[PATCH BUNDLE v9] famfs: Fabric-Attached Memory File System

[PATCH BUNDLE v9] famfs: Fabric-Attached Memory File System

[John Groves]

This is a coordinated patch submission for famfs (Fabric-Attached Memory
File System) across three repositories:

  1. Linux kernel dax (cover + 7 patches) - dax fsdev driver + fuse/famfs 
     integration
  2. Linux kernel fs/fuse - famfs support into fuse. Depends on the dax
     patches

NOTE: the fuse fuse package (second series) depends on the dax patches
(first series).

This bundle does not contain the user space libfuse and ndctl patches;
those will be sent separately to the respective projects.

Each series is posted as a reply to this cover message, with individual
patches replying to their respective series cover.

Overview
--------
Famfs exposes shared memory as a file system. It consumes shared memory
from dax devices and provides memory-mappable files that map directly to
the memory with no page cache involvement. Famfs differs from conventional
file systems in fs-dax mode in that it handles in-memory metadata in a
sharable way (which begins with never caching dirty shared metadata).

Famfs started as a standalone file system [1,2], but the consensus at
LSFMM 2024 and 2025 [3,4] was that it should be ported into fuse.

The key performance requirement is that famfs must resolve mapping faults
without upcalls. This is achieved by fully caching the file-to-devdax
metadata for all active files via two fuse client/server message/response
pairs: GET_FMAP and GET_DAXDEV.

Patch Series Summary
--------------------

Linux Kernel dax (V8, xx patches): New fsdev driver (drivers/dax/fsdev.c)
providing a devdax mode compatible with fs-dax. Devices can be switched
among 'devdax', 'fsdev' and 'system-ram' modes via daxctl or sysfs.


Linux kernel fuse (V8, xx patches: Famfs integration adding GET_FMAP and
GET_DAXDEV messages for caching file-to-dax mappings in the kernel.

Testing
-------
The famfs user space [5] includes comprehensive smoke and unit tests that
exercise all three components together. The ndctl series includes a
dedicated test for famfs mode transitions.

References
----------
[1] https://lore.kernel.org/linux-cxl/cover.1708709155.git.john@groves.net/
[2] https://lore.kernel.org/linux-cxl/cover.1714409084.git.john@groves.net/
[3] https://lwn.net/Articles/983105/ (LSFMM 2024)
[4] https://lwn.net/Articles/1020170/ (LSFMM 2025)
[5] https://famfs.org (famfs user space)
[6] https://lore.kernel.org/linux-cxl/20250703185032.46568-1-john@groves.net/ (V2)
[7] https://lore.kernel.org/linux-fsdevel/20260107153244.64703-1-john@groves.net/T/#m0000d8c00290f48c086b8b176c7525e410f8508c (related ndctl series)
--
John Groves

[PATCH V9 0/8] dax: prepare for famfs

[John Groves]

From: John Groves <john@groves.net>

This patch series along with the bundled patches to fuse are available
as a git tag at [0].

Changes v8 -> v9
- Big clarifying comments in fs/dax.c:dax_folio_reset_order() in response
  to Jonathan's comments/questions
- Added drivers/dax/Kconfig:CONFIG_DEV_DAX_FSDEV to control inclusion of the
  new famfs dax mode (which is in drivers/dax/fsdev.c - bind that to a daxdev
  and you have famfs mode)
- Some trivial refactoring, mostly per Jonathan's comments

Description:

This patch series introduces the required dax support for famfs.
Previous versions of the famfs series included both dax and fuse patches.
This series separates them into separate patch series' (and the fuse
series dependends on this dax series).

The famfs user space code can be found at [1]

Dax Overview:

This series introduces a new "famfs mode" of devdax, whose driver is
drivers/dax/fsdev.c. This driver supports dax_iomap_rw() and
dax_iomap_fault() calls against a character dax instance. A dax device
now can be converted among three modes: 'system-ram', 'devdax' and
'famfs' via daxctl or sysfs (e.g. unbind devdax and bind famfs instead).

In famfs mode, a dax device initializes its pages consistent with the
fsdaxmode of pmem. Raw read/write/mmap are not supported in this mode,
but famfs is happy in this mode - using dax_iomap_rw() for read/write and
dax_iomap_fault() for mmap faults.


Changes v7 -> v8
- dax: Added a devm action to clear folio state when unbinding fsdev.c
  (thanks Allison)
- Added a missing device_lock() in fs_dax_get() (thanks Dave)
- Re-factored some __free blocks for inline declaration
- Used FIELD_PREP where appropriate
- Minor doc edits

Changes v6 -> v7
- Fixed a regression in famfs_interleave_fileofs_to_daxofs() that
  was reported by Intel's kernel test robot
- Added a check in __fsdev_dax_direct_access() for negative return
  from pgoff_to_phys(), which would indicate an out-of-range offset
- Fixed a bug in __famfs_meta_free(), where not all interleaved
  extents were freed
- Added chunksize alignment checks in famfs_fuse_meta_alloc() and
  famfs_interleave_fileofs_to_daxofs() as interleaved chunks must
  be PTE or PMD aligned
- Simplified famfs_file_init_dax() a bit
- Re-ran CM's kernel code review prompts on the entire series and
  fixed several minor issues

Changes v4 -> v5 -> v6
- None. Re-sending due to technical difficulties

Changes v3 [9] -> v4
- The patch "dax: prevent driver unbind while filesystem holds device"
  has been dropped. Dan Williams indicated that the favored behavior is
  for a file system to stop working if an underlying driver is unbound,
  rather than preventing the unbind.
- The patch "famfs_fuse: Famfs mount opt: -o shadow=<shadowpath>" has
  been dropped. Found a way for the famfs user space to do without the
  -o opt (via getxattr).
- Squashed the fs/fuse/Kconfig patch into the first subsequent patch
  that needed the change
  ("famfs_fuse: Basic fuse kernel ABI enablement for famfs")
- Many review comments addressed.
- Addressed minor kerneldoc infractions reported by test robot.

Changes v2 [7] -> v3
- Dax: Completely new fsdev driver (drivers/dax/fsdev.c) replaces the
  dev_dax_iomap modifications to bus.c/device.c. Devdax devices can now
  be switched among 'devdax', 'famfs' and 'system-ram' modes via daxctl
  or sysfs.
- Dax: fsdev uses MEMORY_DEVICE_FS_DAX type and leaves folios at order-0
  (no vmemmap_shift), allowing fs-dax to manage folio lifecycles
  dynamically like pmem does.
- Dax: The "poisoned page" problem is properly fixed via
  fsdev_clear_folio_state(), which clears stale mapping/compound state
  when fsdev binds. The temporary WARN_ON_ONCE workaround in fs/dax.c
  has been removed.
- Dax: Added dax_set_ops() so fsdev can set dax_operations at bind time
  (and clear them on unbind), since the dax_device is created before we
  know which driver will bind.
- Dax: Added custom bind/unbind sysfs handlers; unbind return -EBUSY if a
  filesystem holds the device, preventing unbind while famfs is mounted.
- Fuse: Famfs mounts now require that the fuse server/daemon has
  CAP_SYS_RAWIO because they expose raw memory devices.
- Fuse: Added DAX address_space_operations with noop_dirty_folio since
  famfs is memory-backed with no writeback required.
- Rebased to latest kernels, fully compatible with Alistair Popple
  et. al's recent dax refactoring.
- Ran this series through Chris Mason's code review AI prompts to check
  for issues - several subtle problems found and fixed.
- Dropped RFC status - this version is intended to be mergeable.

Changes v1 [8] -> v2:

- The GET_FMAP message/response has been moved from LOOKUP to OPEN, as
  was the pretty much unanimous consensus.
- Made the response payload to GET_FMAP variable sized (patch 12)
- Dodgy kerneldoc comments cleaned up or removed.
- Fixed memory leak of fc->shadow in patch 11 (thanks Joanne)
- Dropped many pr_debug and pr_notice calls


References

[0] - https://github.com/jagalactic/linux/tree/famfs-v9 (this patch set)
[1] - https://famfs.org (famfs user space)
[2] - https://lore.kernel.org/linux-cxl/cover.1708709155.git.john@groves.net/
[3] - https://lore.kernel.org/linux-cxl/cover.1714409084.git.john@groves.net/
[4] - https://lwn.net/Articles/983105/ (lsfmm 2024)
[5] - https://lwn.net/Articles/1020170/ (lsfmm 2025)
[6] - https://lore.kernel.org/linux-cxl/cover.8068ad144a7eea4a813670301f4d2a86a8e68ec4.1740713401.git-series.apopple@nvidia.com/
[7] - https://lore.kernel.org/linux-fsdevel/20250703185032.46568-1-john@groves.net/ (famfs fuse v2)
[8] - https://lore.kernel.org/linux-fsdevel/20250421013346.32530-1-john@groves.net/ (famfs fuse v1)
[9] - https://lore.kernel.org/linux-fsdevel/20260107153244.64703-1-john@groves.net/T/#mb2c868801be16eca82dab239a1d201628534aea7 (famfs fuse v3)


John Groves (8):
  dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c
  dax: Factor out dax_folio_reset_order() helper
  dax: add fsdev.c driver for fs-dax on character dax
  dax: Save the kva from memremap
  dax: Add dax_operations for use by fs-dax on fsdev dax
  dax: Add dax_set_ops() for setting dax_operations at bind time
  dax: Add fs_dax_get() func to prepare dax for fs-dax usage
  dax: export dax_dev_get()

 MAINTAINERS               |   8 +
 drivers/dax/Kconfig       |  11 ++
 drivers/dax/Makefile      |   2 +
 drivers/dax/bus.c         |  30 +++-
 drivers/dax/bus.h         |   3 +
 drivers/dax/dax-private.h |   3 +
 drivers/dax/device.c      |  23 ---
 drivers/dax/fsdev.c       | 346 ++++++++++++++++++++++++++++++++++++++
 drivers/dax/super.c       | 107 +++++++++++-
 fs/dax.c                  |  75 +++++++--
 include/linux/dax.h       |  20 ++-
 11 files changed, 578 insertions(+), 50 deletions(-)
 create mode 100644 drivers/dax/fsdev.c


base-commit: c369299895a591d96745d6492d4888259b004a9e
-- 
2.53.0

[PATCH V9 1/8] dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

[John Groves]

From: John Groves <john@groves.net>

This function will be used by both device.c and fsdev.c, but both are
loadable modules. Moving to bus.c puts it in core and makes it available
to both.

No code changes - just relocated.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/bus.c    | 24 ++++++++++++++++++++++++
 drivers/dax/device.c | 23 -----------------------
 2 files changed, 24 insertions(+), 23 deletions(-)

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index c94c09622516..e4bd5c9f006c 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -1417,6 +1417,30 @@ static const struct device_type dev_dax_type = {
 	.groups = dax_attribute_groups,
 };
 
+/* see "strong" declaration in tools/testing/nvdimm/dax-dev.c */
+__weak phys_addr_t dax_pgoff_to_phys(struct dev_dax *dev_dax, pgoff_t pgoff,
+			      unsigned long size)
+{
+	int i;
+
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct dev_dax_range *dax_range = &dev_dax->ranges[i];
+		struct range *range = &dax_range->range;
+		unsigned long long pgoff_end;
+		phys_addr_t phys;
+
+		pgoff_end = dax_range->pgoff + PHYS_PFN(range_len(range)) - 1;
+		if (pgoff < dax_range->pgoff || pgoff > pgoff_end)
+			continue;
+		phys = PFN_PHYS(pgoff - dax_range->pgoff) + range->start;
+		if (phys + size - 1 <= range->end)
+			return phys;
+		break;
+	}
+	return -1;
+}
+EXPORT_SYMBOL_GPL(dax_pgoff_to_phys);
+
 static struct dev_dax *__devm_create_dev_dax(struct dev_dax_data *data)
 {
 	struct dax_region *dax_region = data->dax_region;
diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 528e81240c4d..2d2dbfd35e94 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -57,29 +57,6 @@ static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
 			   vma->vm_file, func);
 }
 
-/* see "strong" declaration in tools/testing/nvdimm/dax-dev.c */
-__weak phys_addr_t dax_pgoff_to_phys(struct dev_dax *dev_dax, pgoff_t pgoff,
-		unsigned long size)
-{
-	int i;
-
-	for (i = 0; i < dev_dax->nr_range; i++) {
-		struct dev_dax_range *dax_range = &dev_dax->ranges[i];
-		struct range *range = &dax_range->range;
-		unsigned long long pgoff_end;
-		phys_addr_t phys;
-
-		pgoff_end = dax_range->pgoff + PHYS_PFN(range_len(range)) - 1;
-		if (pgoff < dax_range->pgoff || pgoff > pgoff_end)
-			continue;
-		phys = PFN_PHYS(pgoff - dax_range->pgoff) + range->start;
-		if (phys + size - 1 <= range->end)
-			return phys;
-		break;
-	}
-	return -1;
-}
-
 static void dax_set_mapping(struct vm_fault *vmf, unsigned long pfn,
 			      unsigned long fault_size)
 {
-- 
2.53.0

Re: [PATCH V9 1/8] dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:37:53 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <john@groves.net>
> 
> This function will be used by both device.c and fsdev.c, but both are
> loadable modules. Moving to bus.c puts it in core and makes it available
> to both.
> 
> No code changes - just relocated.
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: John Groves <john@groves.net>
Obviously this is a straight forward code move... But I can't resist
commenting on what is moving  (feel free to ignore! or maybe a follow
up patch if you agree.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> ---
>  drivers/dax/bus.c    | 24 ++++++++++++++++++++++++
>  drivers/dax/device.c | 23 -----------------------
>  2 files changed, 24 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index c94c09622516..e4bd5c9f006c 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1417,6 +1417,30 @@ static const struct device_type dev_dax_type = {
>  	.groups = dax_attribute_groups,
>  };
>  
> +/* see "strong" declaration in tools/testing/nvdimm/dax-dev.c */
> +__weak phys_addr_t dax_pgoff_to_phys(struct dev_dax *dev_dax, pgoff_t pgoff,
> +			      unsigned long size)
> +{
> +	int i;
> +
> +	for (i = 0; i < dev_dax->nr_range; i++) {
Modernize as:

	for (int i = 0; ...

> +		struct dev_dax_range *dax_range = &dev_dax->ranges[i];
> +		struct range *range = &dax_range->range;
> +		unsigned long long pgoff_end;
> +		phys_addr_t phys;
> +
> +		pgoff_end = dax_range->pgoff + PHYS_PFN(range_len(range)) - 1;
> +		if (pgoff < dax_range->pgoff || pgoff > pgoff_end)

We have in_range() (linux/minmax.h) available for these case of a start to start + length - 1
check. I think this is same:

		if (!in_range(pgoff, dax_range->pgoff, PHYS_PFN(range_len(range))) 

> +			continue;
> +		phys = PFN_PHYS(pgoff - dax_range->pgoff) + range->start;
> +		if (phys + size - 1 <= range->end)
> +			return phys;
> +		break;
> +	}
> +	return -1;
> +}
> +EXPORT_SYMBOL_GPL(dax_pgoff_to_phys);
> +

Re: [PATCH V9 1/8] dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

[Ira Weiny]

Jonathan Cameron wrote:
> On Tue, 24 Mar 2026 00:37:53 +0000
> John Groves <john@jagalactic.com> wrote:
> 
> > From: John Groves <john@groves.net>
> > 
> > This function will be used by both device.c and fsdev.c, but both are
> > loadable modules. Moving to bus.c puts it in core and makes it available
> > to both.
> > 
> > No code changes - just relocated.
> > 
> > Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> > Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> > Signed-off-by: John Groves <john@groves.net>
> Obviously this is a straight forward code move... But I can't resist
> commenting on what is moving  (feel free to ignore! or maybe a follow
> up patch if you agree.
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

Added this to the series.  LMK if I missed something.

Ira

---
commit ccc1878ab00178e82108bdd1ece497388a24290b (HEAD -> nvdimm-famfs-dax)
Author: Ira Weiny <ira.weiny@intel.com>
Date:   Tue Mar 24 12:36:19 2026 -0500

    dax: Modernize dax_pgoff_to_phys()

    The patch to move dax_pgoff_to_phys() to bus.c revealed that the
    function could be improved with more modern style and the newer
    in_range() utility function.

    Update it while we are moving it around.

    Link: https://lore.kernel.org/all/20260324141806.000003f7@huawei.com/
    Suggested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
    Signed-off-by: Ira Weiny <ira.weiny@intel.com>

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index e4bd5c9f006c..1b412264bb36 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -1421,16 +1421,12 @@ static const struct device_type dev_dax_type = {
 __weak phys_addr_t dax_pgoff_to_phys(struct dev_dax *dev_dax, pgoff_t pgoff,
                              unsigned long size)
 {
-       int i;
-
-       for (i = 0; i < dev_dax->nr_range; i++) {
+       for (int i = 0; i < dev_dax->nr_range; i++) {
                struct dev_dax_range *dax_range = &dev_dax->ranges[i];
                struct range *range = &dax_range->range;
-               unsigned long long pgoff_end;
                phys_addr_t phys;

-               pgoff_end = dax_range->pgoff + PHYS_PFN(range_len(range)) - 1;
-               if (pgoff < dax_range->pgoff || pgoff > pgoff_end)
+               if (!in_range(pgoff, dax_range->pgoff, PHYS_PFN(range_len(range))))
                        continue;
                phys = PFN_PHYS(pgoff - dax_range->pgoff) + range->start;
                if (phys + size - 1 <= range->end)

Re: [PATCH V9 1/8] dax: move dax_pgoff_to_phys from [drivers/dax/] device.c to bus.c

[Jonathan Cameron]

On Tue, 24 Mar 2026 18:44:21 -0500
Ira Weiny <ira.weiny@intel.com> wrote:

> Jonathan Cameron wrote:
> > On Tue, 24 Mar 2026 00:37:53 +0000
> > John Groves <john@jagalactic.com> wrote:
> >   
> > > From: John Groves <john@groves.net>
> > > 
> > > This function will be used by both device.c and fsdev.c, but both are
> > > loadable modules. Moving to bus.c puts it in core and makes it available
> > > to both.
> > > 
> > > No code changes - just relocated.
> > > 
> > > Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> > > Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> > > Signed-off-by: John Groves <john@groves.net>  
> > Obviously this is a straight forward code move... But I can't resist
> > commenting on what is moving  (feel free to ignore! or maybe a follow
> > up patch if you agree.
> > 
> > Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>  
> 
> Added this to the series.  LMK if I missed something.
LGTM.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
(don't bother adding it unless you are touching the tree for some other reason!)

> 
> Ira
> 
> ---
> commit ccc1878ab00178e82108bdd1ece497388a24290b (HEAD -> nvdimm-famfs-dax)
> Author: Ira Weiny <ira.weiny@intel.com>
> Date:   Tue Mar 24 12:36:19 2026 -0500
> 
>     dax: Modernize dax_pgoff_to_phys()
> 
>     The patch to move dax_pgoff_to_phys() to bus.c revealed that the
>     function could be improved with more modern style and the newer
>     in_range() utility function.
> 
>     Update it while we are moving it around.
> 
>     Link: https://lore.kernel.org/all/20260324141806.000003f7@huawei.com/
>     Suggested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
>     Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index e4bd5c9f006c..1b412264bb36 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1421,16 +1421,12 @@ static const struct device_type dev_dax_type = {
>  __weak phys_addr_t dax_pgoff_to_phys(struct dev_dax *dev_dax, pgoff_t pgoff,
>                               unsigned long size)
>  {
> -       int i;
> -
> -       for (i = 0; i < dev_dax->nr_range; i++) {
> +       for (int i = 0; i < dev_dax->nr_range; i++) {
>                 struct dev_dax_range *dax_range = &dev_dax->ranges[i];
>                 struct range *range = &dax_range->range;
> -               unsigned long long pgoff_end;
>                 phys_addr_t phys;
> 
> -               pgoff_end = dax_range->pgoff + PHYS_PFN(range_len(range)) - 1;
> -               if (pgoff < dax_range->pgoff || pgoff > pgoff_end)
> +               if (!in_range(pgoff, dax_range->pgoff, PHYS_PFN(range_len(range))))
>                         continue;
>                 phys = PFN_PHYS(pgoff - dax_range->pgoff) + range->start;
>                 if (phys + size - 1 <= range->end)
> 

[PATCH V9 2/8] dax: Factor out dax_folio_reset_order() helper

[John Groves]

From: John Groves <John@Groves.net>

Both fs/dax.c:dax_folio_put() and drivers/dax/fsdev.c:
fsdev_clear_folio_state() (the latter coming in the next commit after this
one) contain nearly identical code to reset a compound DAX folio back to
order-0 pages. Factor this out into a shared helper function.

The new dax_folio_reset_order() function:
- Clears the folio's mapping and share count
- Resets compound folio state via folio_reset_order()
- Clears PageHead and compound_head for each sub-page
- Restores the pgmap pointer for each resulting order-0 folio
- Returns the original folio order (for callers that need to advance by
  that many pages)

Two intentional differences from the original dax_folio_put() logic:

1. folio->share is cleared unconditionally. This is correct because the DAX
   subsystem maintains the invariant that share != 0 only when mapping == NULL
   (enforced by dax_folio_make_shared()). dax_folio_put() ensures share has
   reached zero before calling this helper, so the unconditional clear is safe.

2. folio->pgmap is now explicitly restored for order-0 folios. For the
   dax_folio_put() caller this is a no-op (reads and writes back the same
   field). It is intentional for the upcoming fsdev_clear_folio_state()
   caller, which converts previously-compound folios and needs pgmap
   re-established for all pages regardless of order.

This simplifies fsdev_clear_folio_state() from ~50 lines to ~15 lines.

Suggested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 fs/dax.c            | 74 ++++++++++++++++++++++++++++++++++-----------
 include/linux/dax.h |  1 +
 2 files changed, 57 insertions(+), 18 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 289e6254aa30..eba86802a7a7 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -378,6 +378,59 @@ static void dax_folio_make_shared(struct folio *folio)
 	folio->share = 1;
 }
 
+/**
+ * dax_folio_reset_order - Reset a compound DAX folio to order-0 pages
+ * @folio: The folio to reset
+ *
+ * Splits a compound folio back into individual order-0 pages,
+ * clearing compound state and restoring pgmap pointers.
+ *
+ * Returns: the original folio order (0 if already order-0)
+ */
+int dax_folio_reset_order(struct folio *folio)
+{
+	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
+	int order = folio_order(folio);
+
+	/*
+	 * DAX maintains the invariant that folio->share != 0 only when
+	 * folio->mapping == NULL (enforced by dax_folio_make_shared()).
+	 * Equivalently: folio->mapping != NULL implies folio->share == 0.
+	 * Callers ensure share has been decremented to zero before
+	 * calling here, so unconditionally clearing both fields is
+	 * correct.
+	 */
+	folio->mapping = NULL;
+	folio->share = 0;
+
+	if (!order) {
+		/*
+		 * Restore pgmap explicitly even for order-0 folios. For
+		 * the dax_folio_put() caller this is a no-op (same value),
+		 * but fsdev_clear_folio_state() may call this on folios
+		 * that were previously compound and need pgmap
+		 * re-established.
+		 */
+		folio->pgmap = pgmap;
+		return 0;
+	}
+
+	folio_reset_order(folio);
+
+	for (int i = 0; i < (1UL << order); i++) {
+		struct page *page = folio_page(folio, i);
+		struct folio *f = (struct folio *)page;
+
+		ClearPageHead(page);
+		clear_compound_head(page);
+		f->mapping = NULL;
+		f->share = 0;
+		f->pgmap = pgmap;
+	}
+
+	return order;
+}
+
 static inline unsigned long dax_folio_put(struct folio *folio)
 {
 	unsigned long ref;
@@ -391,28 +444,13 @@ static inline unsigned long dax_folio_put(struct folio *folio)
 	if (ref)
 		return ref;
 
-	folio->mapping = NULL;
-	order = folio_order(folio);
-	if (!order)
-		return 0;
-	folio_reset_order(folio);
+	order = dax_folio_reset_order(folio);
 
+	/* Debug check: verify refcounts are zero for all sub-folios */
 	for (i = 0; i < (1UL << order); i++) {
-		struct dev_pagemap *pgmap = page_pgmap(&folio->page);
 		struct page *page = folio_page(folio, i);
-		struct folio *new_folio = (struct folio *)page;
-
-		ClearPageHead(page);
-		clear_compound_head(page);
 
-		new_folio->mapping = NULL;
-		/*
-		 * Reset pgmap which was over-written by
-		 * prep_compound_page().
-		 */
-		new_folio->pgmap = pgmap;
-		new_folio->share = 0;
-		WARN_ON_ONCE(folio_ref_count(new_folio));
+		WARN_ON_ONCE(folio_ref_count((struct folio *)page));
 	}
 
 	return ref;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index bf103f317cac..73cfc1a7c8f1 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -153,6 +153,7 @@ static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
 #if IS_ENABLED(CONFIG_FS_DAX)
 int dax_writeback_mapping_range(struct address_space *mapping,
 		struct dax_device *dax_dev, struct writeback_control *wbc);
+int dax_folio_reset_order(struct folio *folio);
 
 struct page *dax_layout_busy_page(struct address_space *mapping);
 struct page *dax_layout_busy_page_range(struct address_space *mapping, loff_t start, loff_t end);
-- 
2.53.0

Re: [PATCH V9 2/8] dax: Factor out dax_folio_reset_order() helper

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:38:15 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <John@Groves.net>
> 
> Both fs/dax.c:dax_folio_put() and drivers/dax/fsdev.c:
> fsdev_clear_folio_state() (the latter coming in the next commit after this
> one) contain nearly identical code to reset a compound DAX folio back to
> order-0 pages. Factor this out into a shared helper function.
> 
> The new dax_folio_reset_order() function:
> - Clears the folio's mapping and share count
> - Resets compound folio state via folio_reset_order()
> - Clears PageHead and compound_head for each sub-page
> - Restores the pgmap pointer for each resulting order-0 folio
> - Returns the original folio order (for callers that need to advance by
>   that many pages)
> 
> Two intentional differences from the original dax_folio_put() logic:
> 
> 1. folio->share is cleared unconditionally. This is correct because the DAX
>    subsystem maintains the invariant that share != 0 only when mapping == NULL
>    (enforced by dax_folio_make_shared()). dax_folio_put() ensures share has
>    reached zero before calling this helper, so the unconditional clear is safe.
> 
> 2. folio->pgmap is now explicitly restored for order-0 folios. For the
>    dax_folio_put() caller this is a no-op (reads and writes back the same
>    field). It is intentional for the upcoming fsdev_clear_folio_state()
>    caller, which converts previously-compound folios and needs pgmap
>    re-established for all pages regardless of order.
> 
> This simplifies fsdev_clear_folio_state() from ~50 lines to ~15 lines.
> 
> Suggested-by: Jonathan Cameron <jonathan.cameron@huawei.com>
A couple of trivial "if you are respinning" line length of comments
comments inline.
Subject to DAX folk sanity checking the new comments match their
expectations.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>


> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: John Groves <john@groves.net>
> ---
>  fs/dax.c            | 74 ++++++++++++++++++++++++++++++++++-----------
>  include/linux/dax.h |  1 +
>  2 files changed, 57 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 289e6254aa30..eba86802a7a7 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -378,6 +378,59 @@ static void dax_folio_make_shared(struct folio *folio)
>  	folio->share = 1;
>  }
>  
> +/**
> + * dax_folio_reset_order - Reset a compound DAX folio to order-0 pages
> + * @folio: The folio to reset
> + *
> + * Splits a compound folio back into individual order-0 pages,
> + * clearing compound state and restoring pgmap pointers.
> + *
> + * Returns: the original folio order (0 if already order-0)
> + */
> +int dax_folio_reset_order(struct folio *folio)
> +{
> +	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
> +	int order = folio_order(folio);
> +
> +	/*
> +	 * DAX maintains the invariant that folio->share != 0 only when
> +	 * folio->mapping == NULL (enforced by dax_folio_make_shared()).
> +	 * Equivalently: folio->mapping != NULL implies folio->share == 0.
> +	 * Callers ensure share has been decremented to zero before
> +	 * calling here, so unconditionally clearing both fields is
> +	 * correct.

If you happen to spin again, wrap is a bit short of standard 80 chars.
	 * DAX maintains the invariant that folio->share != 0 only when
	 * folio->mapping == NULL (enforced by dax_folio_make_shared()).
	 * Equivalently: folio->mapping != NULL implies folio->share == 0.
	 * Callers ensure share has been decremented to zero before calling here,
	 * so unconditionally clearing both fields is correct.
> +	 */
> +	folio->mapping = NULL;
> +	folio->share = 0;
> +
> +	if (!order) {
> +		/*
> +		 * Restore pgmap explicitly even for order-0 folios. For
> +		 * the dax_folio_put() caller this is a no-op (same value),
> +		 * but fsdev_clear_folio_state() may call this on folios
> +		 * that were previously compound and need pgmap
> +		 * re-established.
> +		 */
		 * Restore pgmap explicitly even for order-0 folios. For the
		 * dax_folio_put() caller this is a no-op (same value), but
		 * fsdev_clear_folio_state() may call this on folios that were
		 * previously compound and need pgmap re-established.
		 */

> +		folio->pgmap = pgmap;
> +		return 0;
> +	}
> +
> +	folio_reset_order(folio);
> +
> +	for (int i = 0; i < (1UL << order); i++) {
> +		struct page *page = folio_page(folio, i);
> +		struct folio *f = (struct folio *)page;
> +
> +		ClearPageHead(page);
> +		clear_compound_head(page);
> +		f->mapping = NULL;
> +		f->share = 0;
> +		f->pgmap = pgmap;
> +	}
> +
> +	return order;
> +}

[PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[John Groves]

From: John Groves <john@groves.net>

The new fsdev driver provides pages/folios initialized compatibly with
fsdax - normal rather than devdax-style refcounting, and starting out
with order-0 folios.

When fsdev binds to a daxdev, it is usually (always?) switching from the
devdax mode (device.c), which pre-initializes compound folios according
to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
folios into a fsdax-compatible state.

A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
dax instance. Accordingly, The fsdev driver does not provide raw mmap -
devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
mmap capability.

In this commit is just the framework, which remaps pages/folios compatibly
with fsdax.

Enabling dax changes:

- bus.h: add DAXDRV_FSDEV_TYPE driver type
- bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
- dax.h: prototype inode_dax(), which fsdev needs

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Gregory Price <gourry@gourry.net>
Signed-off-by: John Groves <john@groves.net>
---
 MAINTAINERS          |   8 ++
 drivers/dax/Kconfig  |  11 ++
 drivers/dax/Makefile |   2 +
 drivers/dax/bus.c    |   4 +
 drivers/dax/bus.h    |   1 +
 drivers/dax/fsdev.c  | 245 +++++++++++++++++++++++++++++++++++++++++++
 fs/dax.c             |   1 +
 7 files changed, 272 insertions(+)
 create mode 100644 drivers/dax/fsdev.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 7d10988cbc62..eedf4cce56ed 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7298,6 +7298,14 @@ L:	linux-cxl@vger.kernel.org
 S:	Supported
 F:	drivers/dax/
 
+DEVICE DIRECT ACCESS (DAX) [fsdev_dax]
+M:	John Groves <jgroves@micron.com>
+M:	John Groves <John@Groves.net>
+L:	nvdimm@lists.linux.dev
+L:	linux-cxl@vger.kernel.org
+S:	Supported
+F:	drivers/dax/fsdev.c
+
 DEVICE FREQUENCY (DEVFREQ)
 M:	MyungJoo Ham <myungjoo.ham@samsung.com>
 M:	Kyungmin Park <kyungmin.park@samsung.com>
diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
index d656e4c0eb84..7051b70980d5 100644
--- a/drivers/dax/Kconfig
+++ b/drivers/dax/Kconfig
@@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
 	depends on DEV_DAX_HMEM && DAX
 	def_bool y
 
+config DEV_DAX_FSDEV
+	tristate "FSDEV DAX: fs-dax compatible devdax driver"
+	depends on DEV_DAX && FS_DAX
+	help
+	  Support fs-dax access to DAX devices via a character device
+	  interface. Unlike device_dax (which pre-initializes compound folios
+	  based on device alignment), this driver leaves folios at order-0 so
+	  that fs-dax filesystems can manage folio order dynamically.
+
+	  Say M if unsure.
+
 config DEV_DAX_KMEM
 	tristate "KMEM DAX: map dax-devices as System-RAM"
 	default DEV_DAX
diff --git a/drivers/dax/Makefile b/drivers/dax/Makefile
index 5ed5c39857c8..ba35bda7abef 100644
--- a/drivers/dax/Makefile
+++ b/drivers/dax/Makefile
@@ -4,11 +4,13 @@ obj-$(CONFIG_DEV_DAX) += device_dax.o
 obj-$(CONFIG_DEV_DAX_KMEM) += kmem.o
 obj-$(CONFIG_DEV_DAX_PMEM) += dax_pmem.o
 obj-$(CONFIG_DEV_DAX_CXL) += dax_cxl.o
+obj-$(CONFIG_DEV_DAX_FSDEV) += fsdev_dax.o
 
 dax-y := super.o
 dax-y += bus.o
 device_dax-y := device.o
 dax_pmem-y := pmem.o
 dax_cxl-y := cxl.o
+fsdev_dax-y := fsdev.o
 
 obj-y += hmem/
diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index e4bd5c9f006c..562e2b06f61a 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -81,6 +81,10 @@ static int dax_match_type(const struct dax_device_driver *dax_drv, struct device
 	    !IS_ENABLED(CONFIG_DEV_DAX_KMEM))
 		return 1;
 
+	/* fsdev driver can also bind to device-type dax devices */
+	if (dax_drv->type == DAXDRV_FSDEV_TYPE && type == DAXDRV_DEVICE_TYPE)
+		return 1;
+
 	return 0;
 }
 
diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
index cbbf64443098..880bdf7e72d7 100644
--- a/drivers/dax/bus.h
+++ b/drivers/dax/bus.h
@@ -31,6 +31,7 @@ struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data);
 enum dax_driver_type {
 	DAXDRV_KMEM_TYPE,
 	DAXDRV_DEVICE_TYPE,
+	DAXDRV_FSDEV_TYPE,
 };
 
 struct dax_device_driver {
diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
new file mode 100644
index 000000000000..8b5c6976ad17
--- /dev/null
+++ b/drivers/dax/fsdev.c
@@ -0,0 +1,245 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright(c) 2026 Micron Technology, Inc. */
+#include <linux/memremap.h>
+#include <linux/pagemap.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/cdev.h>
+#include <linux/slab.h>
+#include <linux/dax.h>
+#include <linux/uio.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include "dax-private.h"
+#include "bus.h"
+
+/*
+ * FS-DAX compatible devdax driver
+ *
+ * Unlike drivers/dax/device.c which pre-initializes compound folios based
+ * on device alignment (via vmemmap_shift), this driver leaves folios
+ * uninitialized similar to pmem. This allows fs-dax filesystems like famfs
+ * to work without needing special handling for pre-initialized folios.
+ *
+ * Key differences from device.c:
+ * - pgmap type is MEMORY_DEVICE_FS_DAX (not MEMORY_DEVICE_GENERIC)
+ * - vmemmap_shift is NOT set (folios remain order-0)
+ * - fs-dax can dynamically create compound folios as needed
+ * - No mmap support - all access is through fs-dax/iomap
+ */
+
+static void fsdev_cdev_del(void *cdev)
+{
+	cdev_del(cdev);
+}
+
+static void fsdev_kill(void *dev_dax)
+{
+	kill_dev_dax(dev_dax);
+}
+
+/*
+ * Page map operations for FS-DAX mode
+ * Similar to fsdax_pagemap_ops in drivers/nvdimm/pmem.c
+ *
+ * Note: folio_free callback is not needed for MEMORY_DEVICE_FS_DAX.
+ * The core mm code in free_zone_device_folio() handles the wake_up_var()
+ * directly for this memory type.
+ */
+static int fsdev_pagemap_memory_failure(struct dev_pagemap *pgmap,
+		unsigned long pfn, unsigned long nr_pages, int mf_flags)
+{
+	struct dev_dax *dev_dax = pgmap->owner;
+	u64 offset = PFN_PHYS(pfn) - dev_dax->ranges[0].range.start;
+	u64 len = nr_pages << PAGE_SHIFT;
+
+	return dax_holder_notify_failure(dev_dax->dax_dev, offset,
+					 len, mf_flags);
+}
+
+static const struct dev_pagemap_ops fsdev_pagemap_ops = {
+	.memory_failure		= fsdev_pagemap_memory_failure,
+};
+
+/*
+ * Clear any stale folio state from pages in the given range.
+ * This is necessary because device_dax pre-initializes compound folios
+ * based on vmemmap_shift, and that state may persist after driver unbind.
+ * Since fsdev_dax uses MEMORY_DEVICE_FS_DAX without vmemmap_shift, fs-dax
+ * expects to find clean order-0 folios that it can build into compound
+ * folios on demand.
+ *
+ * At probe time, no filesystem should be mounted yet, so all mappings
+ * are stale and must be cleared along with compound state.
+ */
+static void fsdev_clear_folio_state(struct dev_dax *dev_dax)
+{
+	for (int i = 0; i < dev_dax->nr_range; i++) {
+		struct range *range = &dev_dax->ranges[i].range;
+		unsigned long pfn = PHYS_PFN(range->start);
+		unsigned long end_pfn = PHYS_PFN(range->end) + 1;
+
+		while (pfn < end_pfn) {
+			struct folio *folio = pfn_folio(pfn);
+			int order = dax_folio_reset_order(folio);
+
+			pfn += 1UL << order;
+		}
+	}
+}
+
+static void fsdev_clear_folio_state_action(void *data)
+{
+	fsdev_clear_folio_state(data);
+}
+
+static int fsdev_open(struct inode *inode, struct file *filp)
+{
+	struct dax_device *dax_dev = inode_dax(inode);
+	struct dev_dax *dev_dax = dax_get_private(dax_dev);
+
+	filp->private_data = dev_dax;
+
+	return 0;
+}
+
+static int fsdev_release(struct inode *inode, struct file *filp)
+{
+	return 0;
+}
+
+static const struct file_operations fsdev_fops = {
+	.llseek = noop_llseek,
+	.owner = THIS_MODULE,
+	.open = fsdev_open,
+	.release = fsdev_release,
+};
+
+static int fsdev_dax_probe(struct dev_dax *dev_dax)
+{
+	struct dax_device *dax_dev = dev_dax->dax_dev;
+	struct device *dev = &dev_dax->dev;
+	struct dev_pagemap *pgmap;
+	struct inode *inode;
+	struct cdev *cdev;
+	void *addr;
+	int rc, i;
+
+	if (static_dev_dax(dev_dax)) {
+		if (dev_dax->nr_range > 1) {
+			dev_warn(dev, "static pgmap / multi-range device conflict\n");
+			return -EINVAL;
+		}
+
+		pgmap = dev_dax->pgmap;
+	} else {
+		size_t pgmap_size;
+
+		if (dev_dax->pgmap) {
+			dev_warn(dev, "dynamic-dax with pre-populated page map\n");
+			return -EINVAL;
+		}
+
+		pgmap_size = struct_size(pgmap, ranges, dev_dax->nr_range - 1);
+		pgmap = devm_kzalloc(dev, pgmap_size, GFP_KERNEL);
+		if (!pgmap)
+			return -ENOMEM;
+
+		pgmap->nr_range = dev_dax->nr_range;
+		dev_dax->pgmap = pgmap;
+
+		for (i = 0; i < dev_dax->nr_range; i++) {
+			struct range *range = &dev_dax->ranges[i].range;
+
+			pgmap->ranges[i] = *range;
+		}
+	}
+
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct range *range = &dev_dax->ranges[i].range;
+
+		if (!devm_request_mem_region(dev, range->start,
+					range_len(range), dev_name(dev))) {
+			dev_warn(dev, "mapping%d: %#llx-%#llx could not reserve range\n",
+				 i, range->start, range->end);
+			return -EBUSY;
+		}
+	}
+
+	/*
+	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
+	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this
+	 * lets fs-dax dynamically build compound folios as needed, similar
+	 * to pmem behavior.
+	 */
+	pgmap->type = MEMORY_DEVICE_FS_DAX;
+	pgmap->ops = &fsdev_pagemap_ops;
+	pgmap->owner = dev_dax;
+
+	addr = devm_memremap_pages(dev, pgmap);
+	if (IS_ERR(addr))
+		return PTR_ERR(addr);
+
+	/*
+	 * Clear any stale compound folio state left over from a previous
+	 * driver (e.g., device_dax with vmemmap_shift). Also register this
+	 * as a devm action so folio state is cleared on unbind, ensuring
+	 * clean pages for subsequent drivers (e.g., kmem for system-ram).
+	 */
+	fsdev_clear_folio_state(dev_dax);
+	rc = devm_add_action_or_reset(dev, fsdev_clear_folio_state_action,
+				      dev_dax);
+	if (rc)
+		return rc;
+
+	/* Detect whether the data is at a non-zero offset into the memory */
+	if (pgmap->range.start != dev_dax->ranges[0].range.start) {
+		u64 phys = dev_dax->ranges[0].range.start;
+		u64 pgmap_phys = dev_dax->pgmap[0].range.start;
+		u64 data_offset = 0;
+
+		if (!WARN_ON(pgmap_phys > phys))
+			data_offset = phys - pgmap_phys;
+
+		pr_debug("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx\n",
+		       __func__, phys, pgmap_phys, data_offset);
+	}
+
+	inode = dax_inode(dax_dev);
+	cdev = inode->i_cdev;
+	cdev_init(cdev, &fsdev_fops);
+	cdev->owner = dev->driver->owner;
+	cdev_set_parent(cdev, &dev->kobj);
+	rc = cdev_add(cdev, dev->devt, 1);
+	if (rc)
+		return rc;
+
+	rc = devm_add_action_or_reset(dev, fsdev_cdev_del, cdev);
+	if (rc)
+		return rc;
+
+	run_dax(dax_dev);
+	return devm_add_action_or_reset(dev, fsdev_kill, dev_dax);
+}
+
+static struct dax_device_driver fsdev_dax_driver = {
+	.probe = fsdev_dax_probe,
+	.type = DAXDRV_FSDEV_TYPE,
+};
+
+static int __init dax_init(void)
+{
+	return dax_driver_register(&fsdev_dax_driver);
+}
+
+static void __exit dax_exit(void)
+{
+	dax_driver_unregister(&fsdev_dax_driver);
+}
+
+MODULE_AUTHOR("John Groves");
+MODULE_DESCRIPTION("FS-DAX Device: fs-dax compatible devdax driver");
+MODULE_LICENSE("GPL");
+module_init(dax_init);
+module_exit(dax_exit);
+MODULE_ALIAS_DAX_DEVICE(0);
diff --git a/fs/dax.c b/fs/dax.c
index eba86802a7a7..b91a2535149a 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -430,6 +430,7 @@ int dax_folio_reset_order(struct folio *folio)
 
 	return order;
 }
+EXPORT_SYMBOL_GPL(dax_folio_reset_order);
 
 static inline unsigned long dax_folio_put(struct folio *folio)
 {
-- 
2.53.0

Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:38:31 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <john@groves.net>
> 
> The new fsdev driver provides pages/folios initialized compatibly with
> fsdax - normal rather than devdax-style refcounting, and starting out
> with order-0 folios.
> 
> When fsdev binds to a daxdev, it is usually (always?) switching from the
> devdax mode (device.c), which pre-initializes compound folios according
> to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> folios into a fsdax-compatible state.
> 
> A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> mmap capability.
> 
> In this commit is just the framework, which remaps pages/folios compatibly
> with fsdax.
> 
> Enabling dax changes:
> 
> - bus.h: add DAXDRV_FSDEV_TYPE driver type
> - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> - dax.h: prototype inode_dax(), which fsdev needs
> 
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: John Groves <john@groves.net>

I was kind of thinking you'd go with a hidden KCONFIG option with default
magic to do the same build condition to you had in the Makefil, but one the
user can opt in or out for is also fine.

Comments on that below. Meh, I think this is better anyway :)

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>



> diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> index d656e4c0eb84..7051b70980d5 100644
> --- a/drivers/dax/Kconfig
> +++ b/drivers/dax/Kconfig
> @@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
>  	depends on DEV_DAX_HMEM && DAX
>  	def_bool y
>  
> +config DEV_DAX_FSDEV
> +	tristate "FSDEV DAX: fs-dax compatible devdax driver"
> +	depends on DEV_DAX && FS_DAX
> +	help
> +	  Support fs-dax access to DAX devices via a character device
> +	  interface. Unlike device_dax (which pre-initializes compound folios
> +	  based on device alignment), this driver leaves folios at order-0 so
> +	  that fs-dax filesystems can manage folio order dynamically.
> +
> +	  Say M if unsure.
Fine like this, but if you wanted to hide it in interests of not
confusing users...

config DEV_DAX_FSDEV
	tristate
	depends on DEV_DAX && FS_DAX
	default DEV_DAX

> +
>  config DEV_DAX_KMEM
>  	tristate "KMEM DAX: map dax-devices as System-RAM"
>  	default DEV_DAX

> +}

Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[John Groves]

On 26/03/24 02:39PM, Jonathan Cameron wrote:
> On Tue, 24 Mar 2026 00:38:31 +0000
> John Groves <john@jagalactic.com> wrote:
> 
> > From: John Groves <john@groves.net>
> > 
> > The new fsdev driver provides pages/folios initialized compatibly with
> > fsdax - normal rather than devdax-style refcounting, and starting out
> > with order-0 folios.
> > 
> > When fsdev binds to a daxdev, it is usually (always?) switching from the
> > devdax mode (device.c), which pre-initializes compound folios according
> > to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> > folios into a fsdax-compatible state.
> > 
> > A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> > dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> > devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> > mmap capability.
> > 
> > In this commit is just the framework, which remaps pages/folios compatibly
> > with fsdax.
> > 
> > Enabling dax changes:
> > 
> > - bus.h: add DAXDRV_FSDEV_TYPE driver type
> > - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> > - dax.h: prototype inode_dax(), which fsdev needs
> > 
> > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > Suggested-by: Gregory Price <gourry@gourry.net>
> > Signed-off-by: John Groves <john@groves.net>
> 
> I was kind of thinking you'd go with a hidden KCONFIG option with default
> magic to do the same build condition to you had in the Makefil, but one the
> user can opt in or out for is also fine.
> 
> Comments on that below. Meh, I think this is better anyway :)
> 
> Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> 
> 
> 
> > diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> > index d656e4c0eb84..7051b70980d5 100644
> > --- a/drivers/dax/Kconfig
> > +++ b/drivers/dax/Kconfig
> > @@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
> >  	depends on DEV_DAX_HMEM && DAX
> >  	def_bool y
> >  
> > +config DEV_DAX_FSDEV
> > +	tristate "FSDEV DAX: fs-dax compatible devdax driver"
> > +	depends on DEV_DAX && FS_DAX
> > +	help
> > +	  Support fs-dax access to DAX devices via a character device
> > +	  interface. Unlike device_dax (which pre-initializes compound folios
> > +	  based on device alignment), this driver leaves folios at order-0 so
> > +	  that fs-dax filesystems can manage folio order dynamically.
> > +
> > +	  Say M if unsure.
> Fine like this, but if you wanted to hide it in interests of not
> confusing users...
> 
> config DEV_DAX_FSDEV
> 	tristate
> 	depends on DEV_DAX && FS_DAX
> 	default DEV_DAX

I like this better. I see no reason not to default to including fsdev.
It does nothing other than frustrating famfs users if it's off - since
building it still has no effect unless you put a daxdev in famfs mode.

Ira, it's kinda in your hands at the moment. Do you feel like making this
change?

> 
> > +
> >  config DEV_DAX_KMEM
> >  	tristate "KMEM DAX: map dax-devices as System-RAM"
> >  	default DEV_DAX
> 
> > +}
> 

Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[Ira Weiny]

John Groves wrote:
> On 26/03/24 02:39PM, Jonathan Cameron wrote:
> > On Tue, 24 Mar 2026 00:38:31 +0000
> > John Groves <john@jagalactic.com> wrote:
> > 
> > > From: John Groves <john@groves.net>
> > > 
> > > The new fsdev driver provides pages/folios initialized compatibly with
> > > fsdax - normal rather than devdax-style refcounting, and starting out
> > > with order-0 folios.
> > > 
> > > When fsdev binds to a daxdev, it is usually (always?) switching from the
> > > devdax mode (device.c), which pre-initializes compound folios according
> > > to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> > > folios into a fsdax-compatible state.
> > > 
> > > A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> > > dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> > > devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> > > mmap capability.
> > > 
> > > In this commit is just the framework, which remaps pages/folios compatibly
> > > with fsdax.
> > > 
> > > Enabling dax changes:
> > > 
> > > - bus.h: add DAXDRV_FSDEV_TYPE driver type
> > > - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> > > - dax.h: prototype inode_dax(), which fsdev needs
> > > 
> > > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > > Suggested-by: Gregory Price <gourry@gourry.net>
> > > Signed-off-by: John Groves <john@groves.net>
> > 
> > I was kind of thinking you'd go with a hidden KCONFIG option with default
> > magic to do the same build condition to you had in the Makefil, but one the
> > user can opt in or out for is also fine.
> > 
> > Comments on that below. Meh, I think this is better anyway :)
> > 
> > Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> > 
> > 
> > 
> > > diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> > > index d656e4c0eb84..7051b70980d5 100644
> > > --- a/drivers/dax/Kconfig
> > > +++ b/drivers/dax/Kconfig
> > > @@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
> > >  	depends on DEV_DAX_HMEM && DAX
> > >  	def_bool y
> > >  
> > > +config DEV_DAX_FSDEV
> > > +	tristate "FSDEV DAX: fs-dax compatible devdax driver"
> > > +	depends on DEV_DAX && FS_DAX
> > > +	help
> > > +	  Support fs-dax access to DAX devices via a character device
> > > +	  interface. Unlike device_dax (which pre-initializes compound folios
> > > +	  based on device alignment), this driver leaves folios at order-0 so
> > > +	  that fs-dax filesystems can manage folio order dynamically.
> > > +
> > > +	  Say M if unsure.
> > Fine like this, but if you wanted to hide it in interests of not
> > confusing users...
> > 
> > config DEV_DAX_FSDEV
> > 	tristate
> > 	depends on DEV_DAX && FS_DAX
> > 	default DEV_DAX
> 
> I like this better. I see no reason not to default to including fsdev.
> It does nothing other than frustrating famfs users if it's off - since
> building it still has no effect unless you put a daxdev in famfs mode.
> 
> Ira, it's kinda in your hands at the moment. Do you feel like making this
> change?

I don't mind making this change.  But we have to deal with the breakage to
current device dax users.

https://lore.kernel.org/all/69c36921255b6_e9d8d1009b@iweiny-mobl.notmuch/

What am I missing?

Ira

Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[John Groves]

On 26/03/25 11:04AM, Ira Weiny wrote:
> John Groves wrote:
> > On 26/03/24 02:39PM, Jonathan Cameron wrote:
> > > On Tue, 24 Mar 2026 00:38:31 +0000
> > > John Groves <john@jagalactic.com> wrote:
> > > 
> > > > From: John Groves <john@groves.net>
> > > > 
> > > > The new fsdev driver provides pages/folios initialized compatibly with
> > > > fsdax - normal rather than devdax-style refcounting, and starting out
> > > > with order-0 folios.
> > > > 
> > > > When fsdev binds to a daxdev, it is usually (always?) switching from the
> > > > devdax mode (device.c), which pre-initializes compound folios according
> > > > to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> > > > folios into a fsdax-compatible state.
> > > > 
> > > > A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> > > > dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> > > > devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> > > > mmap capability.
> > > > 
> > > > In this commit is just the framework, which remaps pages/folios compatibly
> > > > with fsdax.
> > > > 
> > > > Enabling dax changes:
> > > > 
> > > > - bus.h: add DAXDRV_FSDEV_TYPE driver type
> > > > - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> > > > - dax.h: prototype inode_dax(), which fsdev needs
> > > > 
> > > > Suggested-by: Dan Williams <dan.j.williams@intel.com>
> > > > Suggested-by: Gregory Price <gourry@gourry.net>
> > > > Signed-off-by: John Groves <john@groves.net>
> > > 
> > > I was kind of thinking you'd go with a hidden KCONFIG option with default
> > > magic to do the same build condition to you had in the Makefil, but one the
> > > user can opt in or out for is also fine.
> > > 
> > > Comments on that below. Meh, I think this is better anyway :)
> > > 
> > > Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
> > > 
> > > 
> > > 
> > > > diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> > > > index d656e4c0eb84..7051b70980d5 100644
> > > > --- a/drivers/dax/Kconfig
> > > > +++ b/drivers/dax/Kconfig
> > > > @@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
> > > >  	depends on DEV_DAX_HMEM && DAX
> > > >  	def_bool y
> > > >  
> > > > +config DEV_DAX_FSDEV
> > > > +	tristate "FSDEV DAX: fs-dax compatible devdax driver"
> > > > +	depends on DEV_DAX && FS_DAX
> > > > +	help
> > > > +	  Support fs-dax access to DAX devices via a character device
> > > > +	  interface. Unlike device_dax (which pre-initializes compound folios
> > > > +	  based on device alignment), this driver leaves folios at order-0 so
> > > > +	  that fs-dax filesystems can manage folio order dynamically.
> > > > +
> > > > +	  Say M if unsure.
> > > Fine like this, but if you wanted to hide it in interests of not
> > > confusing users...
> > > 
> > > config DEV_DAX_FSDEV
> > > 	tristate
> > > 	depends on DEV_DAX && FS_DAX
> > > 	default DEV_DAX
> > 
> > I like this better. I see no reason not to default to including fsdev.
> > It does nothing other than frustrating famfs users if it's off - since
> > building it still has no effect unless you put a daxdev in famfs mode.
> > 
> > Ira, it's kinda in your hands at the moment. Do you feel like making this
> > change?
> 
> I don't mind making this change.  But we have to deal with the breakage to
> current device dax users.
> 
> https://lore.kernel.org/all/69c36921255b6_e9d8d1009b@iweiny-mobl.notmuch/
> 
> What am I missing?
> 
> Ira

OK, I can reproduce that failure with kernel 7.0.0-rc5 and 
straight ndctl v84. So it's not famfs.

I also studied the verbose logs trying to figure out if famfs
could cause it (while running a famfs kernel and ndctl), but
I don't see it.

Then I tried non-famfs kernel and ndctl and it's the same with
or without famfs kernel and famfs ndctl.

John

Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[Dave Jiang]

On 3/23/26 5:38 PM, John Groves wrote:
> From: John Groves <john@groves.net>
> 
> The new fsdev driver provides pages/folios initialized compatibly with
> fsdax - normal rather than devdax-style refcounting, and starting out
> with order-0 folios.
> 
> When fsdev binds to a daxdev, it is usually (always?) switching from the
> devdax mode (device.c), which pre-initializes compound folios according
> to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> folios into a fsdax-compatible state.
> 
> A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> mmap capability.
> 
> In this commit is just the framework, which remaps pages/folios compatibly
> with fsdax.
> 
> Enabling dax changes:
> 
> - bus.h: add DAXDRV_FSDEV_TYPE driver type
> - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> - dax.h: prototype inode_dax(), which fsdev needs
> 
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: John Groves <john@groves.net>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>


> ---
>  MAINTAINERS          |   8 ++
>  drivers/dax/Kconfig  |  11 ++
>  drivers/dax/Makefile |   2 +
>  drivers/dax/bus.c    |   4 +
>  drivers/dax/bus.h    |   1 +
>  drivers/dax/fsdev.c  | 245 +++++++++++++++++++++++++++++++++++++++++++
>  fs/dax.c             |   1 +
>  7 files changed, 272 insertions(+)
>  create mode 100644 drivers/dax/fsdev.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 7d10988cbc62..eedf4cce56ed 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -7298,6 +7298,14 @@ L:	linux-cxl@vger.kernel.org
>  S:	Supported
>  F:	drivers/dax/
>  
> +DEVICE DIRECT ACCESS (DAX) [fsdev_dax]
> +M:	John Groves <jgroves@micron.com>
> +M:	John Groves <John@Groves.net>
> +L:	nvdimm@lists.linux.dev
> +L:	linux-cxl@vger.kernel.org
> +S:	Supported
> +F:	drivers/dax/fsdev.c
> +
>  DEVICE FREQUENCY (DEVFREQ)
>  M:	MyungJoo Ham <myungjoo.ham@samsung.com>
>  M:	Kyungmin Park <kyungmin.park@samsung.com>
> diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> index d656e4c0eb84..7051b70980d5 100644
> --- a/drivers/dax/Kconfig
> +++ b/drivers/dax/Kconfig
> @@ -61,6 +61,17 @@ config DEV_DAX_HMEM_DEVICES
>  	depends on DEV_DAX_HMEM && DAX
>  	def_bool y
>  
> +config DEV_DAX_FSDEV
> +	tristate "FSDEV DAX: fs-dax compatible devdax driver"
> +	depends on DEV_DAX && FS_DAX
> +	help
> +	  Support fs-dax access to DAX devices via a character device
> +	  interface. Unlike device_dax (which pre-initializes compound folios
> +	  based on device alignment), this driver leaves folios at order-0 so
> +	  that fs-dax filesystems can manage folio order dynamically.
> +
> +	  Say M if unsure.
> +
>  config DEV_DAX_KMEM
>  	tristate "KMEM DAX: map dax-devices as System-RAM"
>  	default DEV_DAX
> diff --git a/drivers/dax/Makefile b/drivers/dax/Makefile
> index 5ed5c39857c8..ba35bda7abef 100644
> --- a/drivers/dax/Makefile
> +++ b/drivers/dax/Makefile
> @@ -4,11 +4,13 @@ obj-$(CONFIG_DEV_DAX) += device_dax.o
>  obj-$(CONFIG_DEV_DAX_KMEM) += kmem.o
>  obj-$(CONFIG_DEV_DAX_PMEM) += dax_pmem.o
>  obj-$(CONFIG_DEV_DAX_CXL) += dax_cxl.o
> +obj-$(CONFIG_DEV_DAX_FSDEV) += fsdev_dax.o
>  
>  dax-y := super.o
>  dax-y += bus.o
>  device_dax-y := device.o
>  dax_pmem-y := pmem.o
>  dax_cxl-y := cxl.o
> +fsdev_dax-y := fsdev.o
>  
>  obj-y += hmem/
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index e4bd5c9f006c..562e2b06f61a 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -81,6 +81,10 @@ static int dax_match_type(const struct dax_device_driver *dax_drv, struct device
>  	    !IS_ENABLED(CONFIG_DEV_DAX_KMEM))
>  		return 1;
>  
> +	/* fsdev driver can also bind to device-type dax devices */
> +	if (dax_drv->type == DAXDRV_FSDEV_TYPE && type == DAXDRV_DEVICE_TYPE)
> +		return 1;
> +
>  	return 0;
>  }
>  
> diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
> index cbbf64443098..880bdf7e72d7 100644
> --- a/drivers/dax/bus.h
> +++ b/drivers/dax/bus.h
> @@ -31,6 +31,7 @@ struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data);
>  enum dax_driver_type {
>  	DAXDRV_KMEM_TYPE,
>  	DAXDRV_DEVICE_TYPE,
> +	DAXDRV_FSDEV_TYPE,
>  };
>  
>  struct dax_device_driver {
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> new file mode 100644
> index 000000000000..8b5c6976ad17
> --- /dev/null
> +++ b/drivers/dax/fsdev.c
> @@ -0,0 +1,245 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright(c) 2026 Micron Technology, Inc. */
> +#include <linux/memremap.h>
> +#include <linux/pagemap.h>
> +#include <linux/module.h>
> +#include <linux/device.h>
> +#include <linux/cdev.h>
> +#include <linux/slab.h>
> +#include <linux/dax.h>
> +#include <linux/uio.h>
> +#include <linux/fs.h>
> +#include <linux/mm.h>
> +#include "dax-private.h"
> +#include "bus.h"
> +
> +/*
> + * FS-DAX compatible devdax driver
> + *
> + * Unlike drivers/dax/device.c which pre-initializes compound folios based
> + * on device alignment (via vmemmap_shift), this driver leaves folios
> + * uninitialized similar to pmem. This allows fs-dax filesystems like famfs
> + * to work without needing special handling for pre-initialized folios.
> + *
> + * Key differences from device.c:
> + * - pgmap type is MEMORY_DEVICE_FS_DAX (not MEMORY_DEVICE_GENERIC)
> + * - vmemmap_shift is NOT set (folios remain order-0)
> + * - fs-dax can dynamically create compound folios as needed
> + * - No mmap support - all access is through fs-dax/iomap
> + */
> +
> +static void fsdev_cdev_del(void *cdev)
> +{
> +	cdev_del(cdev);
> +}
> +
> +static void fsdev_kill(void *dev_dax)
> +{
> +	kill_dev_dax(dev_dax);
> +}
> +
> +/*
> + * Page map operations for FS-DAX mode
> + * Similar to fsdax_pagemap_ops in drivers/nvdimm/pmem.c
> + *
> + * Note: folio_free callback is not needed for MEMORY_DEVICE_FS_DAX.
> + * The core mm code in free_zone_device_folio() handles the wake_up_var()
> + * directly for this memory type.
> + */
> +static int fsdev_pagemap_memory_failure(struct dev_pagemap *pgmap,
> +		unsigned long pfn, unsigned long nr_pages, int mf_flags)
> +{
> +	struct dev_dax *dev_dax = pgmap->owner;
> +	u64 offset = PFN_PHYS(pfn) - dev_dax->ranges[0].range.start;
> +	u64 len = nr_pages << PAGE_SHIFT;
> +
> +	return dax_holder_notify_failure(dev_dax->dax_dev, offset,
> +					 len, mf_flags);
> +}
> +
> +static const struct dev_pagemap_ops fsdev_pagemap_ops = {
> +	.memory_failure		= fsdev_pagemap_memory_failure,
> +};
> +
> +/*
> + * Clear any stale folio state from pages in the given range.
> + * This is necessary because device_dax pre-initializes compound folios
> + * based on vmemmap_shift, and that state may persist after driver unbind.
> + * Since fsdev_dax uses MEMORY_DEVICE_FS_DAX without vmemmap_shift, fs-dax
> + * expects to find clean order-0 folios that it can build into compound
> + * folios on demand.
> + *
> + * At probe time, no filesystem should be mounted yet, so all mappings
> + * are stale and must be cleared along with compound state.
> + */
> +static void fsdev_clear_folio_state(struct dev_dax *dev_dax)
> +{
> +	for (int i = 0; i < dev_dax->nr_range; i++) {
> +		struct range *range = &dev_dax->ranges[i].range;
> +		unsigned long pfn = PHYS_PFN(range->start);
> +		unsigned long end_pfn = PHYS_PFN(range->end) + 1;
> +
> +		while (pfn < end_pfn) {
> +			struct folio *folio = pfn_folio(pfn);
> +			int order = dax_folio_reset_order(folio);
> +
> +			pfn += 1UL << order;
> +		}
> +	}
> +}
> +
> +static void fsdev_clear_folio_state_action(void *data)
> +{
> +	fsdev_clear_folio_state(data);
> +}
> +
> +static int fsdev_open(struct inode *inode, struct file *filp)
> +{
> +	struct dax_device *dax_dev = inode_dax(inode);
> +	struct dev_dax *dev_dax = dax_get_private(dax_dev);
> +
> +	filp->private_data = dev_dax;
> +
> +	return 0;
> +}
> +
> +static int fsdev_release(struct inode *inode, struct file *filp)
> +{
> +	return 0;
> +}
> +
> +static const struct file_operations fsdev_fops = {
> +	.llseek = noop_llseek,
> +	.owner = THIS_MODULE,
> +	.open = fsdev_open,
> +	.release = fsdev_release,
> +};
> +
> +static int fsdev_dax_probe(struct dev_dax *dev_dax)
> +{
> +	struct dax_device *dax_dev = dev_dax->dax_dev;
> +	struct device *dev = &dev_dax->dev;
> +	struct dev_pagemap *pgmap;
> +	struct inode *inode;
> +	struct cdev *cdev;
> +	void *addr;
> +	int rc, i;
> +
> +	if (static_dev_dax(dev_dax)) {
> +		if (dev_dax->nr_range > 1) {
> +			dev_warn(dev, "static pgmap / multi-range device conflict\n");
> +			return -EINVAL;
> +		}
> +
> +		pgmap = dev_dax->pgmap;
> +	} else {
> +		size_t pgmap_size;
> +
> +		if (dev_dax->pgmap) {
> +			dev_warn(dev, "dynamic-dax with pre-populated page map\n");
> +			return -EINVAL;
> +		}
> +
> +		pgmap_size = struct_size(pgmap, ranges, dev_dax->nr_range - 1);
> +		pgmap = devm_kzalloc(dev, pgmap_size, GFP_KERNEL);
> +		if (!pgmap)
> +			return -ENOMEM;
> +
> +		pgmap->nr_range = dev_dax->nr_range;
> +		dev_dax->pgmap = pgmap;
> +
> +		for (i = 0; i < dev_dax->nr_range; i++) {
> +			struct range *range = &dev_dax->ranges[i].range;
> +
> +			pgmap->ranges[i] = *range;
> +		}
> +	}
> +
> +	for (i = 0; i < dev_dax->nr_range; i++) {
> +		struct range *range = &dev_dax->ranges[i].range;
> +
> +		if (!devm_request_mem_region(dev, range->start,
> +					range_len(range), dev_name(dev))) {
> +			dev_warn(dev, "mapping%d: %#llx-%#llx could not reserve range\n",
> +				 i, range->start, range->end);
> +			return -EBUSY;
> +		}
> +	}
> +
> +	/*
> +	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
> +	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this
> +	 * lets fs-dax dynamically build compound folios as needed, similar
> +	 * to pmem behavior.
> +	 */
> +	pgmap->type = MEMORY_DEVICE_FS_DAX;
> +	pgmap->ops = &fsdev_pagemap_ops;
> +	pgmap->owner = dev_dax;
> +
> +	addr = devm_memremap_pages(dev, pgmap);
> +	if (IS_ERR(addr))
> +		return PTR_ERR(addr);
> +
> +	/*
> +	 * Clear any stale compound folio state left over from a previous
> +	 * driver (e.g., device_dax with vmemmap_shift). Also register this
> +	 * as a devm action so folio state is cleared on unbind, ensuring
> +	 * clean pages for subsequent drivers (e.g., kmem for system-ram).
> +	 */
> +	fsdev_clear_folio_state(dev_dax);
> +	rc = devm_add_action_or_reset(dev, fsdev_clear_folio_state_action,
> +				      dev_dax);
> +	if (rc)
> +		return rc;
> +
> +	/* Detect whether the data is at a non-zero offset into the memory */
> +	if (pgmap->range.start != dev_dax->ranges[0].range.start) {
> +		u64 phys = dev_dax->ranges[0].range.start;
> +		u64 pgmap_phys = dev_dax->pgmap[0].range.start;
> +		u64 data_offset = 0;
> +
> +		if (!WARN_ON(pgmap_phys > phys))
> +			data_offset = phys - pgmap_phys;
> +
> +		pr_debug("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx\n",
> +		       __func__, phys, pgmap_phys, data_offset);
> +	}
> +
> +	inode = dax_inode(dax_dev);
> +	cdev = inode->i_cdev;
> +	cdev_init(cdev, &fsdev_fops);
> +	cdev->owner = dev->driver->owner;
> +	cdev_set_parent(cdev, &dev->kobj);
> +	rc = cdev_add(cdev, dev->devt, 1);
> +	if (rc)
> +		return rc;
> +
> +	rc = devm_add_action_or_reset(dev, fsdev_cdev_del, cdev);
> +	if (rc)
> +		return rc;
> +
> +	run_dax(dax_dev);
> +	return devm_add_action_or_reset(dev, fsdev_kill, dev_dax);
> +}
> +
> +static struct dax_device_driver fsdev_dax_driver = {
> +	.probe = fsdev_dax_probe,
> +	.type = DAXDRV_FSDEV_TYPE,
> +};
> +
> +static int __init dax_init(void)
> +{
> +	return dax_driver_register(&fsdev_dax_driver);
> +}
> +
> +static void __exit dax_exit(void)
> +{
> +	dax_driver_unregister(&fsdev_dax_driver);
> +}
> +
> +MODULE_AUTHOR("John Groves");
> +MODULE_DESCRIPTION("FS-DAX Device: fs-dax compatible devdax driver");
> +MODULE_LICENSE("GPL");
> +module_init(dax_init);
> +module_exit(dax_exit);
> +MODULE_ALIAS_DAX_DEVICE(0);
> diff --git a/fs/dax.c b/fs/dax.c
> index eba86802a7a7..b91a2535149a 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -430,6 +430,7 @@ int dax_folio_reset_order(struct folio *folio)
>  
>  	return order;
>  }
> +EXPORT_SYMBOL_GPL(dax_folio_reset_order);
>  
>  static inline unsigned long dax_folio_put(struct folio *folio)
>  {

Re: [PATCH V9 3/8] dax: add fsdev.c driver for fs-dax on character dax

[Ira Weiny]

John Groves wrote:
> From: John Groves <john@groves.net>
> 
> The new fsdev driver provides pages/folios initialized compatibly with
> fsdax - normal rather than devdax-style refcounting, and starting out
> with order-0 folios.
> 
> When fsdev binds to a daxdev, it is usually (always?) switching from the
> devdax mode (device.c), which pre-initializes compound folios according
> to its alignment. Fsdev uses fsdev_clear_folio_state() to switch the
> folios into a fsdax-compatible state.
> 
> A side effect of this is that raw mmap doesn't (can't?) work on an fsdev
> dax instance. Accordingly, The fsdev driver does not provide raw mmap -
> devices must be put in 'devdax' mode (drivers/dax/device.c) to get raw
> mmap capability.
> 
> In this commit is just the framework, which remaps pages/folios compatibly
> with fsdax.
> 
> Enabling dax changes:
> 
> - bus.h: add DAXDRV_FSDEV_TYPE driver type
> - bus.c: allow DAXDRV_FSDEV_TYPE drivers to bind to daxdevs
> - dax.h: prototype inode_dax(), which fsdev needs
> 
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Suggested-by: Gregory Price <gourry@gourry.net>
> Signed-off-by: John Groves <john@groves.net>
> ---

[snip]

> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index e4bd5c9f006c..562e2b06f61a 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -81,6 +81,10 @@ static int dax_match_type(const struct dax_device_driver *dax_drv, struct device
>  	    !IS_ENABLED(CONFIG_DEV_DAX_KMEM))
>  		return 1;
>  
> +	/* fsdev driver can also bind to device-type dax devices */
> +	if (dax_drv->type == DAXDRV_FSDEV_TYPE && type == DAXDRV_DEVICE_TYPE)
> +		return 1;
> +

In building up a topic branch for this I notice that this breaks the
device-dax test.

1/1 ndctl:dax / device-dax FAIL             0.19s   (exit status 250 or signal 122 SIGinvalid)
...
Ok:                 0
Expected Fail:      0
Fail:               1
Unexpected Pass:    0
Skipped:            0
Timeout:            0

Have you run this series with all the ndctl cxl selftests?

How exactly is this supposed to ensure the fsdev driver does not bind to
a regular dax device?

This can be fixed by reloading the fsdev module thus pushing that driver
later in the driver list...  So we need to come up with a more reliable
method here.

I'm holding off pushing a topic branch for the time being.

Ira

>  	return 0;
>  }
>  

[snip]

[PATCH V9 4/8] dax: Save the kva from memremap

[John Groves]

From: John Groves <john@groves.net>

Save the kva from memremap because we need it for iomap rw support.

Prior to famfs, there were no iomap users of /dev/dax - so the virtual
address from memremap was not needed.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/dax-private.h | 2 ++
 drivers/dax/fsdev.c       | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
index c6ae27c982f4..7a3727d76a68 100644
--- a/drivers/dax/dax-private.h
+++ b/drivers/dax/dax-private.h
@@ -69,6 +69,7 @@ struct dev_dax_range {
  * data while the device is activated in the driver.
  * @region: parent region
  * @dax_dev: core dax functionality
+ * @virt_addr: kva from memremap; used by fsdev_dax
  * @align: alignment of this instance
  * @target_node: effective numa node if dev_dax memory range is onlined
  * @dyn_id: is this a dynamic or statically created instance
@@ -83,6 +84,7 @@ struct dev_dax_range {
 struct dev_dax {
 	struct dax_region *region;
 	struct dax_device *dax_dev;
+	void *virt_addr;
 	unsigned int align;
 	int target_node;
 	bool dyn_id;
diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index 8b5c6976ad17..c75478d3d548 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -121,6 +121,7 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
 	struct device *dev = &dev_dax->dev;
 	struct dev_pagemap *pgmap;
 	struct inode *inode;
+	u64 data_offset = 0;
 	struct cdev *cdev;
 	void *addr;
 	int rc, i;
@@ -196,7 +197,6 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
 	if (pgmap->range.start != dev_dax->ranges[0].range.start) {
 		u64 phys = dev_dax->ranges[0].range.start;
 		u64 pgmap_phys = dev_dax->pgmap[0].range.start;
-		u64 data_offset = 0;
 
 		if (!WARN_ON(pgmap_phys > phys))
 			data_offset = phys - pgmap_phys;
@@ -204,6 +204,7 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
 		pr_debug("%s: offset detected phys=%llx pgmap_phys=%llx offset=%llx\n",
 		       __func__, phys, pgmap_phys, data_offset);
 	}
+	dev_dax->virt_addr = addr + data_offset;
 
 	inode = dax_inode(dax_dev);
 	cdev = inode->i_cdev;
-- 
2.53.0

Re: [PATCH V9 4/8] dax: Save the kva from memremap

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:38:48 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <john@groves.net>
> 
> Save the kva from memremap because we need it for iomap rw support.
> 
> Prior to famfs, there were no iomap users of /dev/dax - so the virtual
> address from memremap was not needed.
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: John Groves <john@groves.net>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

[PATCH V9 5/8] dax: Add dax_operations for use by fs-dax on fsdev dax

[John Groves]

From: John Groves <John@Groves.net>

fsdev: Add dax_operations for use by famfs.

This replicates the functionality from drivers/nvdimm/pmem.c that
conventional fs-dax file systems (e.g. xfs) use to support dax
read/write/mmap to a daxdev - without which famfs can't sit atop a
daxdev.

- These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
- fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
  newly stored as dev_dax->virt_addr by dev_dax_probe().
- The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
  for read/write (dax_iomap_rw())
- fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
  tested yet. I'm looking for suggestions as to how to test those.
- dax-private.h: add dev_dax->cached_size, which fsdev needs to
  remember. The dev_dax size cannot change while a driver is bound
  (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
  at probe time allows fsdev's direct_access path can use it without
  acquiring dax_dev_rwsem (which isn't exported anyway).

Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/dax-private.h |  1 +
 drivers/dax/fsdev.c       | 84 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
index 7a3727d76a68..ee8f3af8387f 100644
--- a/drivers/dax/dax-private.h
+++ b/drivers/dax/dax-private.h
@@ -85,6 +85,7 @@ struct dev_dax {
 	struct dax_region *region;
 	struct dax_device *dax_dev;
 	void *virt_addr;
+	u64 cached_size;
 	unsigned int align;
 	int target_node;
 	bool dyn_id;
diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index c75478d3d548..be3d2b0e8418 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -28,6 +28,85 @@
  * - No mmap support - all access is through fs-dax/iomap
  */
 
+static void fsdev_write_dax(void *pmem_addr, struct page *page,
+		unsigned int off, unsigned int len)
+{
+	while (len) {
+		void *mem = kmap_local_page(page);
+		unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
+
+		memcpy_flushcache(pmem_addr, mem + off, chunk);
+		kunmap_local(mem);
+		len -= chunk;
+		off = 0;
+		page++;
+		pmem_addr += chunk;
+	}
+}
+
+static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
+			long nr_pages, enum dax_access_mode mode, void **kaddr,
+			unsigned long *pfn)
+{
+	struct dev_dax *dev_dax = dax_get_private(dax_dev);
+	size_t size = nr_pages << PAGE_SHIFT;
+	size_t offset = pgoff << PAGE_SHIFT;
+	void *virt_addr = dev_dax->virt_addr + offset;
+	phys_addr_t phys;
+	unsigned long local_pfn;
+
+	phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
+	if (phys == -1) {
+		dev_dbg(&dev_dax->dev,
+			"pgoff (%#lx) out of range\n", pgoff);
+		return -EFAULT;
+	}
+
+	if (kaddr)
+		*kaddr = virt_addr;
+
+	local_pfn = PHYS_PFN(phys);
+	if (pfn)
+		*pfn = local_pfn;
+
+	/*
+	 * Use cached_size which was computed at probe time. The size cannot
+	 * change while the driver is bound (resize returns -EBUSY).
+	 */
+	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
+}
+
+static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
+			pgoff_t pgoff, size_t nr_pages)
+{
+	void *kaddr;
+
+	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
+	__fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
+	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
+	return 0;
+}
+
+static long fsdev_dax_direct_access(struct dax_device *dax_dev,
+		  pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
+		  void **kaddr, unsigned long *pfn)
+{
+	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
+					 kaddr, pfn);
+}
+
+static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
+		void *addr, size_t bytes, struct iov_iter *i)
+{
+	return _copy_from_iter_flushcache(addr, bytes, i);
+}
+
+static const struct dax_operations dev_dax_ops = {
+	.direct_access = fsdev_dax_direct_access,
+	.zero_page_range = fsdev_dax_zero_page_range,
+	.recovery_write = fsdev_dax_recovery_write,
+};
+
 static void fsdev_cdev_del(void *cdev)
 {
 	cdev_del(cdev);
@@ -167,6 +246,11 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
 		}
 	}
 
+	/* Cache size now; it cannot change while driver is bound */
+	dev_dax->cached_size = 0;
+	for (i = 0; i < dev_dax->nr_range; i++)
+		dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
+
 	/*
 	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
 	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this
-- 
2.53.0

Re: [PATCH V9 5/8] dax: Add dax_operations for use by fs-dax on fsdev dax

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:39:04 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <John@Groves.net>
> 
> fsdev: Add dax_operations for use by famfs.
> 
> This replicates the functionality from drivers/nvdimm/pmem.c that
> conventional fs-dax file systems (e.g. xfs) use to support dax
> read/write/mmap to a daxdev - without which famfs can't sit atop a
> daxdev.
> 
> - These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> - fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
>   newly stored as dev_dax->virt_addr by dev_dax_probe().
> - The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
>   for read/write (dax_iomap_rw())
> - fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
>   tested yet. I'm looking for suggestions as to how to test those.
> - dax-private.h: add dev_dax->cached_size, which fsdev needs to
>   remember. The dev_dax size cannot change while a driver is bound
>   (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
>   at probe time allows fsdev's direct_access path can use it without
>   acquiring dax_dev_rwsem (which isn't exported anyway).
> 
> Signed-off-by: John Groves <john@groves.net>
The indent of trailing parameter lines is very random in here.
Pick a style and stick to it.  Few other trivial things inline.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>


> ---
>  drivers/dax/dax-private.h |  1 +
>  drivers/dax/fsdev.c       | 84 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 85 insertions(+)

> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index c75478d3d548..be3d2b0e8418 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c

> +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> +			long nr_pages, enum dax_access_mode mode, void **kaddr,
> +			unsigned long *pfn)
> +{
> +	struct dev_dax *dev_dax = dax_get_private(dax_dev);
> +	size_t size = nr_pages << PAGE_SHIFT;
> +	size_t offset = pgoff << PAGE_SHIFT;
> +	void *virt_addr = dev_dax->virt_addr + offset;
> +	phys_addr_t phys;
> +	unsigned long local_pfn;
> +
> +	phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
> +	if (phys == -1) {
> +		dev_dbg(&dev_dax->dev,
> +			"pgoff (%#lx) out of range\n", pgoff);
> +		return -EFAULT;
> +	}
> +
> +	if (kaddr)
> +		*kaddr = virt_addr;
> +
> +	local_pfn = PHYS_PFN(phys);
Trivial but if !pfn, local_pfn not used so...

	if (pfn)
		*pfn = PHYS_PFN(phys);

Obviously ignore this if it becomes used in some later patch.

> +	if (pfn)
> +		*pfn = local_pfn;
> +
> +	/*
> +	 * Use cached_size which was computed at probe time. The size cannot
> +	 * change while the driver is bound (resize returns -EBUSY).
Might be worth capturing somewhere in code that using the value from
probe means you don't need locking.
> +	 */
> +	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
> +}
> +
> +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> +			pgoff_t pgoff, size_t nr_pages)
Three tabs
> +{
> +	void *kaddr;
> +
> +	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> +	__fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> +	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> +	return 0;
> +}
> +
> +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> +		  pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,

Why that indent?  Two tabs and a couple of spaces...
Either two tabs, or align after (

> +		  void **kaddr, unsigned long *pfn)
> +{
> +	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> +					 kaddr, pfn);
> +}
> +
> +static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *i)
two tabs....
> +{
> +	return _copy_from_iter_flushcache(addr, bytes, i);
> +}
> +
> +static const struct dax_operations dev_dax_ops = {
> +	.direct_access = fsdev_dax_direct_access,
> +	.zero_page_range = fsdev_dax_zero_page_range,
> +	.recovery_write = fsdev_dax_recovery_write,
> +};

Re: [PATCH V9 5/8] dax: Add dax_operations for use by fs-dax on fsdev dax

[Dave Jiang]

On 3/23/26 5:39 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> fsdev: Add dax_operations for use by famfs.
> 
> This replicates the functionality from drivers/nvdimm/pmem.c that
> conventional fs-dax file systems (e.g. xfs) use to support dax
> read/write/mmap to a daxdev - without which famfs can't sit atop a
> daxdev.
> 
> - These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> - fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
>   newly stored as dev_dax->virt_addr by dev_dax_probe().
> - The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
>   for read/write (dax_iomap_rw())
> - fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
>   tested yet. I'm looking for suggestions as to how to test those.
> - dax-private.h: add dev_dax->cached_size, which fsdev needs to
>   remember. The dev_dax size cannot change while a driver is bound
>   (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
>   at probe time allows fsdev's direct_access path can use it without
>   acquiring dax_dev_rwsem (which isn't exported anyway).
> 
> Signed-off-by: John Groves <john@groves.net>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>


> ---
>  drivers/dax/dax-private.h |  1 +
>  drivers/dax/fsdev.c       | 84 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 85 insertions(+)
> 
> diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
> index 7a3727d76a68..ee8f3af8387f 100644
> --- a/drivers/dax/dax-private.h
> +++ b/drivers/dax/dax-private.h
> @@ -85,6 +85,7 @@ struct dev_dax {
>  	struct dax_region *region;
>  	struct dax_device *dax_dev;
>  	void *virt_addr;
> +	u64 cached_size;
>  	unsigned int align;
>  	int target_node;
>  	bool dyn_id;
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index c75478d3d548..be3d2b0e8418 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c
> @@ -28,6 +28,85 @@
>   * - No mmap support - all access is through fs-dax/iomap
>   */
>  
> +static void fsdev_write_dax(void *pmem_addr, struct page *page,
> +		unsigned int off, unsigned int len)
> +{
> +	while (len) {
> +		void *mem = kmap_local_page(page);
> +		unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
> +
> +		memcpy_flushcache(pmem_addr, mem + off, chunk);
> +		kunmap_local(mem);
> +		len -= chunk;
> +		off = 0;
> +		page++;
> +		pmem_addr += chunk;
> +	}
> +}
> +
> +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> +			long nr_pages, enum dax_access_mode mode, void **kaddr,
> +			unsigned long *pfn)
> +{
> +	struct dev_dax *dev_dax = dax_get_private(dax_dev);
> +	size_t size = nr_pages << PAGE_SHIFT;
> +	size_t offset = pgoff << PAGE_SHIFT;
> +	void *virt_addr = dev_dax->virt_addr + offset;
> +	phys_addr_t phys;
> +	unsigned long local_pfn;
> +
> +	phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
> +	if (phys == -1) {
> +		dev_dbg(&dev_dax->dev,
> +			"pgoff (%#lx) out of range\n", pgoff);
> +		return -EFAULT;
> +	}
> +
> +	if (kaddr)
> +		*kaddr = virt_addr;
> +
> +	local_pfn = PHYS_PFN(phys);
> +	if (pfn)
> +		*pfn = local_pfn;
> +
> +	/*
> +	 * Use cached_size which was computed at probe time. The size cannot
> +	 * change while the driver is bound (resize returns -EBUSY).
> +	 */
> +	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
> +}
> +
> +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> +			pgoff_t pgoff, size_t nr_pages)
> +{
> +	void *kaddr;
> +
> +	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> +	__fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> +	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> +	return 0;
> +}
> +
> +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> +		  pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
> +		  void **kaddr, unsigned long *pfn)
> +{
> +	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> +					 kaddr, pfn);
> +}
> +
> +static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *i)
> +{
> +	return _copy_from_iter_flushcache(addr, bytes, i);
> +}
> +
> +static const struct dax_operations dev_dax_ops = {
> +	.direct_access = fsdev_dax_direct_access,
> +	.zero_page_range = fsdev_dax_zero_page_range,
> +	.recovery_write = fsdev_dax_recovery_write,
> +};
> +
>  static void fsdev_cdev_del(void *cdev)
>  {
>  	cdev_del(cdev);
> @@ -167,6 +246,11 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
>  		}
>  	}
>  
> +	/* Cache size now; it cannot change while driver is bound */
> +	dev_dax->cached_size = 0;
> +	for (i = 0; i < dev_dax->nr_range; i++)
> +		dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
> +
>  	/*
>  	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
>  	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this

Re: [PATCH V9 5/8] dax: Add dax_operations for use by fs-dax on fsdev dax

[Dave Jiang]

On 3/23/26 5:39 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> fsdev: Add dax_operations for use by famfs.
> 
> This replicates the functionality from drivers/nvdimm/pmem.c that
> conventional fs-dax file systems (e.g. xfs) use to support dax
> read/write/mmap to a daxdev - without which famfs can't sit atop a
> daxdev.
> 
> - These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> - fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
>   newly stored as dev_dax->virt_addr by dev_dax_probe().
> - The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
>   for read/write (dax_iomap_rw())
> - fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
>   tested yet. I'm looking for suggestions as to how to test those.
> - dax-private.h: add dev_dax->cached_size, which fsdev needs to
>   remember. The dev_dax size cannot change while a driver is bound
>   (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
>   at probe time allows fsdev's direct_access path can use it without
>   acquiring dax_dev_rwsem (which isn't exported anyway).
> 
> Signed-off-by: John Groves <john@groves.net>
> ---
>  drivers/dax/dax-private.h |  1 +
>  drivers/dax/fsdev.c       | 84 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 85 insertions(+)
> 
> diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
> index 7a3727d76a68..ee8f3af8387f 100644
> --- a/drivers/dax/dax-private.h
> +++ b/drivers/dax/dax-private.h
> @@ -85,6 +85,7 @@ struct dev_dax {
>  	struct dax_region *region;
>  	struct dax_device *dax_dev;
>  	void *virt_addr;
> +	u64 cached_size;

Just caught this. Need a kdoc entry.

DJ

>  	unsigned int align;
>  	int target_node;
>  	bool dyn_id;
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index c75478d3d548..be3d2b0e8418 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c
> @@ -28,6 +28,85 @@
>   * - No mmap support - all access is through fs-dax/iomap
>   */
>  
> +static void fsdev_write_dax(void *pmem_addr, struct page *page,
> +		unsigned int off, unsigned int len)
> +{
> +	while (len) {
> +		void *mem = kmap_local_page(page);
> +		unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
> +
> +		memcpy_flushcache(pmem_addr, mem + off, chunk);
> +		kunmap_local(mem);
> +		len -= chunk;
> +		off = 0;
> +		page++;
> +		pmem_addr += chunk;
> +	}
> +}
> +
> +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> +			long nr_pages, enum dax_access_mode mode, void **kaddr,
> +			unsigned long *pfn)
> +{
> +	struct dev_dax *dev_dax = dax_get_private(dax_dev);
> +	size_t size = nr_pages << PAGE_SHIFT;
> +	size_t offset = pgoff << PAGE_SHIFT;
> +	void *virt_addr = dev_dax->virt_addr + offset;
> +	phys_addr_t phys;
> +	unsigned long local_pfn;
> +
> +	phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
> +	if (phys == -1) {
> +		dev_dbg(&dev_dax->dev,
> +			"pgoff (%#lx) out of range\n", pgoff);
> +		return -EFAULT;
> +	}
> +
> +	if (kaddr)
> +		*kaddr = virt_addr;
> +
> +	local_pfn = PHYS_PFN(phys);
> +	if (pfn)
> +		*pfn = local_pfn;
> +
> +	/*
> +	 * Use cached_size which was computed at probe time. The size cannot
> +	 * change while the driver is bound (resize returns -EBUSY).
> +	 */
> +	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
> +}
> +
> +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> +			pgoff_t pgoff, size_t nr_pages)
> +{
> +	void *kaddr;
> +
> +	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> +	__fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> +	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> +	return 0;
> +}
> +
> +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> +		  pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
> +		  void **kaddr, unsigned long *pfn)
> +{
> +	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> +					 kaddr, pfn);
> +}
> +
> +static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *i)
> +{
> +	return _copy_from_iter_flushcache(addr, bytes, i);
> +}
> +
> +static const struct dax_operations dev_dax_ops = {
> +	.direct_access = fsdev_dax_direct_access,
> +	.zero_page_range = fsdev_dax_zero_page_range,
> +	.recovery_write = fsdev_dax_recovery_write,
> +};
> +
>  static void fsdev_cdev_del(void *cdev)
>  {
>  	cdev_del(cdev);
> @@ -167,6 +246,11 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
>  		}
>  	}
>  
> +	/* Cache size now; it cannot change while driver is bound */
> +	dev_dax->cached_size = 0;
> +	for (i = 0; i < dev_dax->nr_range; i++)
> +		dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
> +
>  	/*
>  	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
>  	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this

Re: [PATCH V9 5/8] dax: Add dax_operations for use by fs-dax on fsdev dax

[Dave Jiang]

On 3/23/26 5:39 PM, John Groves wrote:
> From: John Groves <John@Groves.net>
> 
> fsdev: Add dax_operations for use by famfs.
> 
> This replicates the functionality from drivers/nvdimm/pmem.c that
> conventional fs-dax file systems (e.g. xfs) use to support dax
> read/write/mmap to a daxdev - without which famfs can't sit atop a
> daxdev.
> 
> - These methods are based on pmem_dax_ops from drivers/nvdimm/pmem.c
> - fsdev_dax_direct_access() returns the hpa, pfn and kva. The kva was
>   newly stored as dev_dax->virt_addr by dev_dax_probe().
> - The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
>   for read/write (dax_iomap_rw())
> - fsdev_dax_recovery_write() and dev_dax_zero_page_range() have not been
>   tested yet. I'm looking for suggestions as to how to test those.
> - dax-private.h: add dev_dax->cached_size, which fsdev needs to
>   remember. The dev_dax size cannot change while a driver is bound
>   (dev_dax_resize returns -EBUSY if dev->driver is set). Caching the size
>   at probe time allows fsdev's direct_access path can use it without
>   acquiring dax_dev_rwsem (which isn't exported anyway).
> 
> Signed-off-by: John Groves <john@groves.net>

Couple nits below while I'm stealing code from you.

> ---
>  drivers/dax/dax-private.h |  1 +
>  drivers/dax/fsdev.c       | 84 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 85 insertions(+)
> 
> diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
> index 7a3727d76a68..ee8f3af8387f 100644
> --- a/drivers/dax/dax-private.h
> +++ b/drivers/dax/dax-private.h
> @@ -85,6 +85,7 @@ struct dev_dax {
>  	struct dax_region *region;
>  	struct dax_device *dax_dev;
>  	void *virt_addr;
> +	u64 cached_size;
>  	unsigned int align;
>  	int target_node;
>  	bool dyn_id;
> diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
> index c75478d3d548..be3d2b0e8418 100644
> --- a/drivers/dax/fsdev.c
> +++ b/drivers/dax/fsdev.c
> @@ -28,6 +28,85 @@
>   * - No mmap support - all access is through fs-dax/iomap
>   */
>  
> +static void fsdev_write_dax(void *pmem_addr, struct page *page,

addr instead of pmem_addr? copy pasta error?

> +		unsigned int off, unsigned int len)
> +{
> +	while (len) {
> +		void *mem = kmap_local_page(page);
> +		unsigned int chunk = min_t(unsigned int, len, PAGE_SIZE - off);
> +
> +		memcpy_flushcache(pmem_addr, mem + off, chunk);
> +		kunmap_local(mem);
> +		len -= chunk;
> +		off = 0;
> +		page++;
> +		pmem_addr += chunk;
> +	}
> +}
> +
> +static long __fsdev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> +			long nr_pages, enum dax_access_mode mode, void **kaddr,
> +			unsigned long *pfn)
> +{
> +	struct dev_dax *dev_dax = dax_get_private(dax_dev);
> +	size_t size = nr_pages << PAGE_SHIFT;
> +	size_t offset = pgoff << PAGE_SHIFT;
> +	void *virt_addr = dev_dax->virt_addr + offset;
> +	phys_addr_t phys;
> +	unsigned long local_pfn;
> +
> +	phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);

you can use 'size' instead here since it's previously computed already.

DJ

> +	if (phys == -1) {
> +		dev_dbg(&dev_dax->dev,
> +			"pgoff (%#lx) out of range\n", pgoff);
> +		return -EFAULT;
> +	}
> +
> +	if (kaddr)
> +		*kaddr = virt_addr;
> +
> +	local_pfn = PHYS_PFN(phys);
> +	if (pfn)
> +		*pfn = local_pfn;
> +
> +	/*
> +	 * Use cached_size which was computed at probe time. The size cannot
> +	 * change while the driver is bound (resize returns -EBUSY).
> +	 */
> +	return PHYS_PFN(min(size, dev_dax->cached_size - offset));
> +}
> +
> +static int fsdev_dax_zero_page_range(struct dax_device *dax_dev,
> +			pgoff_t pgoff, size_t nr_pages)
> +{
> +	void *kaddr;
> +
> +	WARN_ONCE(nr_pages > 1, "%s: nr_pages > 1\n", __func__);
> +	__fsdev_dax_direct_access(dax_dev, pgoff, 1, DAX_ACCESS, &kaddr, NULL);
> +	fsdev_write_dax(kaddr, ZERO_PAGE(0), 0, PAGE_SIZE);
> +	return 0;
> +}
> +
> +static long fsdev_dax_direct_access(struct dax_device *dax_dev,
> +		  pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
> +		  void **kaddr, unsigned long *pfn)
> +{
> +	return __fsdev_dax_direct_access(dax_dev, pgoff, nr_pages, mode,
> +					 kaddr, pfn);
> +}
> +
> +static size_t fsdev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
> +		void *addr, size_t bytes, struct iov_iter *i)
> +{
> +	return _copy_from_iter_flushcache(addr, bytes, i);
> +}
> +
> +static const struct dax_operations dev_dax_ops = {
> +	.direct_access = fsdev_dax_direct_access,
> +	.zero_page_range = fsdev_dax_zero_page_range,
> +	.recovery_write = fsdev_dax_recovery_write,
> +};
> +
>  static void fsdev_cdev_del(void *cdev)
>  {
>  	cdev_del(cdev);
> @@ -167,6 +246,11 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
>  		}
>  	}
>  
> +	/* Cache size now; it cannot change while driver is bound */
> +	dev_dax->cached_size = 0;
> +	for (i = 0; i < dev_dax->nr_range; i++)
> +		dev_dax->cached_size += range_len(&dev_dax->ranges[i].range);
> +
>  	/*
>  	 * Use MEMORY_DEVICE_FS_DAX without setting vmemmap_shift, leaving
>  	 * folios at order-0. Unlike device.c (MEMORY_DEVICE_GENERIC), this

[PATCH V9 6/8] dax: Add dax_set_ops() for setting dax_operations at bind time

[John Groves]

From: John Groves <John@Groves.net>

Add a new dax_set_ops() function that allows drivers to set the
dax_operations after the dax_device has been allocated. This is needed
for fsdev_dax where the operations need to be set during probe and
cleared during unbind.

The fsdev driver uses devm_add_action_or_reset() for cleanup consistency,
avoiding the complexity of mixing devm-managed resources with manual
cleanup in a remove() callback. This ensures cleanup happens automatically
in the correct reverse order when the device is unbound.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/fsdev.c | 16 ++++++++++++++++
 drivers/dax/super.c | 38 +++++++++++++++++++++++++++++++++++++-
 include/linux/dax.h |  1 +
 3 files changed, 54 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index be3d2b0e8418..4a5b25515726 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -117,6 +117,13 @@ static void fsdev_kill(void *dev_dax)
 	kill_dev_dax(dev_dax);
 }
 
+static void fsdev_clear_ops(void *data)
+{
+	struct dev_dax *dev_dax = data;
+
+	dax_set_ops(dev_dax->dax_dev, NULL);
+}
+
 /*
  * Page map operations for FS-DAX mode
  * Similar to fsdax_pagemap_ops in drivers/nvdimm/pmem.c
@@ -303,6 +310,15 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
 	if (rc)
 		return rc;
 
+	/* Set the dax operations for fs-dax access path */
+	rc = dax_set_ops(dax_dev, &dev_dax_ops);
+	if (rc)
+		return rc;
+
+	rc = devm_add_action_or_reset(dev, fsdev_clear_ops, dev_dax);
+	if (rc)
+		return rc;
+
 	run_dax(dax_dev);
 	return devm_add_action_or_reset(dev, fsdev_kill, dev_dax);
 }
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index c00b9dff4a06..ba0b4cd18a77 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -157,6 +157,9 @@ long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 	if (!dax_alive(dax_dev))
 		return -ENXIO;
 
+	if (!dax_dev->ops)
+		return -EOPNOTSUPP;
+
 	if (nr_pages < 0)
 		return -EINVAL;
 
@@ -207,6 +210,10 @@ int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
 
 	if (!dax_alive(dax_dev))
 		return -ENXIO;
+
+	if (!dax_dev->ops)
+		return -EOPNOTSUPP;
+
 	/*
 	 * There are no callers that want to zero more than one page as of now.
 	 * Once users are there, this check can be removed after the
@@ -223,7 +230,7 @@ EXPORT_SYMBOL_GPL(dax_zero_page_range);
 size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
 		void *addr, size_t bytes, struct iov_iter *iter)
 {
-	if (!dax_dev->ops->recovery_write)
+	if (!dax_dev->ops || !dax_dev->ops->recovery_write)
 		return 0;
 	return dax_dev->ops->recovery_write(dax_dev, pgoff, addr, bytes, iter);
 }
@@ -307,6 +314,35 @@ void set_dax_nomc(struct dax_device *dax_dev)
 }
 EXPORT_SYMBOL_GPL(set_dax_nomc);
 
+/**
+ * dax_set_ops - set the dax_operations for a dax_device
+ * @dax_dev: the dax_device to configure
+ * @ops: the operations to set (may be NULL to clear)
+ *
+ * This allows drivers to set the dax_operations after the dax_device
+ * has been allocated. This is needed when the device is created before
+ * the driver that needs specific ops is bound (e.g., fsdev_dax binding
+ * to a dev_dax created by hmem).
+ *
+ * When setting non-NULL ops, fails if ops are already set (returns -EBUSY).
+ * When clearing ops (NULL), always succeeds.
+ *
+ * Return: 0 on success, -EBUSY if ops already set
+ */
+int dax_set_ops(struct dax_device *dax_dev, const struct dax_operations *ops)
+{
+	if (ops) {
+		/* Setting ops: fail if already set */
+		if (cmpxchg(&dax_dev->ops, NULL, ops) != NULL)
+			return -EBUSY;
+	} else {
+		/* Clearing ops: always allowed */
+		dax_dev->ops = NULL;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dax_set_ops);
+
 bool dax_alive(struct dax_device *dax_dev)
 {
 	lockdep_assert_held(&dax_srcu);
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 73cfc1a7c8f1..b19bfe0c2fd1 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -243,6 +243,7 @@ static inline void dax_break_layout_final(struct inode *inode)
 
 bool dax_alive(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);
+int dax_set_ops(struct dax_device *dax_dev, const struct dax_operations *ops);
 long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		enum dax_access_mode mode, void **kaddr, unsigned long *pfn);
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
-- 
2.53.0

Re: [PATCH V9 6/8] dax: Add dax_set_ops() for setting dax_operations at bind time

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:39:16 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <John@Groves.net>
> 
> Add a new dax_set_ops() function that allows drivers to set the
> dax_operations after the dax_device has been allocated. This is needed
> for fsdev_dax where the operations need to be set during probe and
> cleared during unbind.
> 
> The fsdev driver uses devm_add_action_or_reset() for cleanup consistency,
> avoiding the complexity of mixing devm-managed resources with manual
> cleanup in a remove() callback. This ensures cleanup happens automatically
> in the correct reverse order when the device is unbound.
> 
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: John Groves <john@groves.net>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

[PATCH V9 7/8] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

[John Groves]

From: John Groves <john@groves.net>

The fs_dax_get() function should be called by fs-dax file systems after
opening a fsdev dax device. This adds holder_operations, which provides
a memory failure callback path and effects exclusivity between callers
of fs_dax_get().

fs_dax_get() is specific to fsdev_dax, so it checks the driver type
(which required touching bus.[ch]). fs_dax_get() fails if fsdev_dax is
not bound to the memory.

This function serves the same role as fs_dax_get_by_bdev(), which dax
file systems call after opening the pmem block device.

This can't be located in fsdev.c because struct dax_device is opaque
there.

This will be called by fs/fuse/famfs.c in a subsequent commit.

Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/bus.c   |  2 --
 drivers/dax/bus.h   |  2 ++
 drivers/dax/super.c | 66 ++++++++++++++++++++++++++++++++++++++++++++-
 include/linux/dax.h | 17 +++++++++---
 4 files changed, 80 insertions(+), 7 deletions(-)

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 562e2b06f61a..8a8710a8234e 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -39,8 +39,6 @@ static int dax_bus_uevent(const struct device *dev, struct kobj_uevent_env *env)
 	return add_uevent_var(env, "MODALIAS=" DAX_DEVICE_MODALIAS_FMT, 0);
 }
 
-#define to_dax_drv(__drv)	container_of_const(__drv, struct dax_device_driver, drv)
-
 static struct dax_id *__dax_match_id(const struct dax_device_driver *dax_drv,
 		const char *dev_name)
 {
diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
index 880bdf7e72d7..dc6f112ac4a4 100644
--- a/drivers/dax/bus.h
+++ b/drivers/dax/bus.h
@@ -42,6 +42,8 @@ struct dax_device_driver {
 	void (*remove)(struct dev_dax *dev);
 };
 
+#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driver, drv)
+
 int __dax_driver_register(struct dax_device_driver *dax_drv,
 		struct module *module, const char *mod_name);
 #define dax_driver_register(driver) \
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index ba0b4cd18a77..d4ab60c406bf 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -14,6 +14,7 @@
 #include <linux/fs.h>
 #include <linux/cacheinfo.h>
 #include "dax-private.h"
+#include "bus.h"
 
 /**
  * struct dax_device - anchor object for dax services
@@ -111,6 +112,10 @@ struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
 }
 EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
 
+#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
+
+#if IS_ENABLED(CONFIG_FS_DAX)
+
 void fs_put_dax(struct dax_device *dax_dev, void *holder)
 {
 	if (dax_dev && holder &&
@@ -119,7 +124,66 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
 	put_dax(dax_dev);
 }
 EXPORT_SYMBOL_GPL(fs_put_dax);
-#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
+
+/**
+ * fs_dax_get() - get ownership of a devdax via holder/holder_ops
+ *
+ * fs-dax file systems call this function to prepare to use a devdax device for
+ * fsdax. This is like fs_dax_get_by_bdev(), but the caller already has struct
+ * dev_dax (and there is no bdev). The holder makes this exclusive.
+ *
+ * @dax_dev: dev to be prepared for fs-dax usage
+ * @holder: filesystem or mapped device inside the dax_device
+ * @hops: operations for the inner holder
+ *
+ * Returns: 0 on success, <0 on failure
+ */
+int fs_dax_get(struct dax_device *dax_dev, void *holder,
+	const struct dax_holder_operations *hops)
+{
+	struct dev_dax *dev_dax;
+	struct dax_device_driver *dax_drv;
+	int id;
+
+	id = dax_read_lock();
+	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode)) {
+		dax_read_unlock(id);
+		return -ENODEV;
+	}
+	dax_read_unlock(id);
+
+	/* Verify the device is bound to fsdev_dax driver */
+	dev_dax = dax_get_private(dax_dev);
+	if (!dev_dax) {
+		iput(&dax_dev->inode);
+		return -ENODEV;
+	}
+
+	device_lock(&dev_dax->dev);
+	if (!dev_dax->dev.driver) {
+		device_unlock(&dev_dax->dev);
+		iput(&dax_dev->inode);
+		return -ENODEV;
+	}
+	dax_drv = to_dax_drv(dev_dax->dev.driver);
+	if (dax_drv->type != DAXDRV_FSDEV_TYPE) {
+		device_unlock(&dev_dax->dev);
+		iput(&dax_dev->inode);
+		return -EOPNOTSUPP;
+	}
+	device_unlock(&dev_dax->dev);
+
+	if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
+		iput(&dax_dev->inode);
+		return -EBUSY;
+	}
+
+	dax_dev->holder_ops = hops;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fs_dax_get);
+#endif /* CONFIG_FS_DAX */
 
 enum dax_device_flags {
 	/* !alive + rcu grace period == no new operations / mappings */
diff --git a/include/linux/dax.h b/include/linux/dax.h
index b19bfe0c2fd1..bf37b9a982f3 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -130,7 +130,6 @@ int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk);
 void dax_remove_host(struct gendisk *disk);
 struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
 		void *holder, const struct dax_holder_operations *ops);
-void fs_put_dax(struct dax_device *dax_dev, void *holder);
 #else
 static inline int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk)
 {
@@ -145,12 +144,13 @@ static inline struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev,
 {
 	return NULL;
 }
-static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
-{
-}
 #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
 
 #if IS_ENABLED(CONFIG_FS_DAX)
+void fs_put_dax(struct dax_device *dax_dev, void *holder);
+int fs_dax_get(struct dax_device *dax_dev, void *holder,
+	       const struct dax_holder_operations *hops);
+struct dax_device *inode_dax(struct inode *inode);
 int dax_writeback_mapping_range(struct address_space *mapping,
 		struct dax_device *dax_dev, struct writeback_control *wbc);
 int dax_folio_reset_order(struct folio *folio);
@@ -164,6 +164,15 @@ dax_entry_t dax_lock_mapping_entry(struct address_space *mapping,
 void dax_unlock_mapping_entry(struct address_space *mapping,
 		unsigned long index, dax_entry_t cookie);
 #else
+static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
+{
+}
+
+static inline int fs_dax_get(struct dax_device *dax_dev, void *holder,
+			     const struct dax_holder_operations *hops)
+{
+	return -EOPNOTSUPP;
+}
 static inline struct page *dax_layout_busy_page(struct address_space *mapping)
 {
 	return NULL;
-- 
2.53.0

Re: [PATCH V9 7/8] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:39:31 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <john@groves.net>
> 
> The fs_dax_get() function should be called by fs-dax file systems after
> opening a fsdev dax device. This adds holder_operations, which provides
> a memory failure callback path and effects exclusivity between callers
> of fs_dax_get().
> 
> fs_dax_get() is specific to fsdev_dax, so it checks the driver type
> (which required touching bus.[ch]). fs_dax_get() fails if fsdev_dax is
> not bound to the memory.
> 
> This function serves the same role as fs_dax_get_by_bdev(), which dax
> file systems call after opening the pmem block device.
> 
> This can't be located in fsdev.c because struct dax_device is opaque
> there.
> 
> This will be called by fs/fuse/famfs.c in a subsequent commit.
> 
> Signed-off-by: John Groves <john@groves.net>
Hi John,

Looks like a stray header change  - see inline.

With that tidied up.
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

>  #define dax_driver_register(driver) \
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index ba0b4cd18a77..d4ab60c406bf 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c

> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index b19bfe0c2fd1..bf37b9a982f3 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h

>  #if IS_ENABLED(CONFIG_FS_DAX)
> +void fs_put_dax(struct dax_device *dax_dev, void *holder);
> +int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +	       const struct dax_holder_operations *hops);
> +struct dax_device *inode_dax(struct inode *inode);

What's this? Not used in this patch and not stubbed.
It's in drivers/dax/dax-private.h already and given I assume code builds
before this patch (and it's not used in patch 8) then presumably it doesn't
need to be here.

I got suspicious due to the lack of stub rather indicating something differnt
form the other two.

>  int dax_writeback_mapping_range(struct address_space *mapping,
>  		struct dax_device *dax_dev, struct writeback_control *wbc);
>  int dax_folio_reset_order(struct folio *folio);
> @@ -164,6 +164,15 @@ dax_entry_t dax_lock_mapping_entry(struct address_space *mapping,
>  void dax_unlock_mapping_entry(struct address_space *mapping,
>  		unsigned long index, dax_entry_t cookie);
>  #else
> +static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
> +{
> +}
> +
> +static inline int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +			     const struct dax_holder_operations *hops)
> +{
> +	return -EOPNOTSUPP;
> +}
>  static inline struct page *dax_layout_busy_page(struct address_space *mapping)
>  {
>  	return NULL;

Re: [PATCH V9 7/8] dax: Add fs_dax_get() func to prepare dax for fs-dax usage

[Dave Jiang]

On 3/23/26 5:39 PM, John Groves wrote:
> From: John Groves <john@groves.net>
> 
> The fs_dax_get() function should be called by fs-dax file systems after
> opening a fsdev dax device. This adds holder_operations, which provides
> a memory failure callback path and effects exclusivity between callers
> of fs_dax_get().
> 
> fs_dax_get() is specific to fsdev_dax, so it checks the driver type
> (which required touching bus.[ch]). fs_dax_get() fails if fsdev_dax is
> not bound to the memory.
> 
> This function serves the same role as fs_dax_get_by_bdev(), which dax
> file systems call after opening the pmem block device.
> 
> This can't be located in fsdev.c because struct dax_device is opaque
> there.
> 
> This will be called by fs/fuse/famfs.c in a subsequent commit.
> 
> Signed-off-by: John Groves <john@groves.net>

Reviewed-by: Dave Jiang <dave.jiang@intel.com>

> ---
>  drivers/dax/bus.c   |  2 --
>  drivers/dax/bus.h   |  2 ++
>  drivers/dax/super.c | 66 ++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/dax.h | 17 +++++++++---
>  4 files changed, 80 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 562e2b06f61a..8a8710a8234e 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -39,8 +39,6 @@ static int dax_bus_uevent(const struct device *dev, struct kobj_uevent_env *env)
>  	return add_uevent_var(env, "MODALIAS=" DAX_DEVICE_MODALIAS_FMT, 0);
>  }
>  
> -#define to_dax_drv(__drv)	container_of_const(__drv, struct dax_device_driver, drv)
> -
>  static struct dax_id *__dax_match_id(const struct dax_device_driver *dax_drv,
>  		const char *dev_name)
>  {
> diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
> index 880bdf7e72d7..dc6f112ac4a4 100644
> --- a/drivers/dax/bus.h
> +++ b/drivers/dax/bus.h
> @@ -42,6 +42,8 @@ struct dax_device_driver {
>  	void (*remove)(struct dev_dax *dev);
>  };
>  
> +#define to_dax_drv(__drv) container_of_const(__drv, struct dax_device_driver, drv)
> +
>  int __dax_driver_register(struct dax_device_driver *dax_drv,
>  		struct module *module, const char *mod_name);
>  #define dax_driver_register(driver) \
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index ba0b4cd18a77..d4ab60c406bf 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -14,6 +14,7 @@
>  #include <linux/fs.h>
>  #include <linux/cacheinfo.h>
>  #include "dax-private.h"
> +#include "bus.h"
>  
>  /**
>   * struct dax_device - anchor object for dax services
> @@ -111,6 +112,10 @@ struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
>  }
>  EXPORT_SYMBOL_GPL(fs_dax_get_by_bdev);
>  
> +#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> +
> +#if IS_ENABLED(CONFIG_FS_DAX)
> +
>  void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  {
>  	if (dax_dev && holder &&
> @@ -119,7 +124,66 @@ void fs_put_dax(struct dax_device *dax_dev, void *holder)
>  	put_dax(dax_dev);
>  }
>  EXPORT_SYMBOL_GPL(fs_put_dax);
> -#endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
> +
> +/**
> + * fs_dax_get() - get ownership of a devdax via holder/holder_ops
> + *
> + * fs-dax file systems call this function to prepare to use a devdax device for
> + * fsdax. This is like fs_dax_get_by_bdev(), but the caller already has struct
> + * dev_dax (and there is no bdev). The holder makes this exclusive.
> + *
> + * @dax_dev: dev to be prepared for fs-dax usage
> + * @holder: filesystem or mapped device inside the dax_device
> + * @hops: operations for the inner holder
> + *
> + * Returns: 0 on success, <0 on failure
> + */
> +int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +	const struct dax_holder_operations *hops)
> +{
> +	struct dev_dax *dev_dax;
> +	struct dax_device_driver *dax_drv;
> +	int id;
> +
> +	id = dax_read_lock();
> +	if (!dax_dev || !dax_alive(dax_dev) || !igrab(&dax_dev->inode)) {
> +		dax_read_unlock(id);
> +		return -ENODEV;
> +	}
> +	dax_read_unlock(id);
> +
> +	/* Verify the device is bound to fsdev_dax driver */
> +	dev_dax = dax_get_private(dax_dev);
> +	if (!dev_dax) {
> +		iput(&dax_dev->inode);
> +		return -ENODEV;
> +	}
> +
> +	device_lock(&dev_dax->dev);
> +	if (!dev_dax->dev.driver) {
> +		device_unlock(&dev_dax->dev);
> +		iput(&dax_dev->inode);
> +		return -ENODEV;
> +	}
> +	dax_drv = to_dax_drv(dev_dax->dev.driver);
> +	if (dax_drv->type != DAXDRV_FSDEV_TYPE) {
> +		device_unlock(&dev_dax->dev);
> +		iput(&dax_dev->inode);
> +		return -EOPNOTSUPP;
> +	}
> +	device_unlock(&dev_dax->dev);
> +
> +	if (cmpxchg(&dax_dev->holder_data, NULL, holder)) {
> +		iput(&dax_dev->inode);
> +		return -EBUSY;
> +	}
> +
> +	dax_dev->holder_ops = hops;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(fs_dax_get);
> +#endif /* CONFIG_FS_DAX */
>  
>  enum dax_device_flags {
>  	/* !alive + rcu grace period == no new operations / mappings */
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index b19bfe0c2fd1..bf37b9a982f3 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -130,7 +130,6 @@ int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk);
>  void dax_remove_host(struct gendisk *disk);
>  struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev, u64 *start_off,
>  		void *holder, const struct dax_holder_operations *ops);
> -void fs_put_dax(struct dax_device *dax_dev, void *holder);
>  #else
>  static inline int dax_add_host(struct dax_device *dax_dev, struct gendisk *disk)
>  {
> @@ -145,12 +144,13 @@ static inline struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev,
>  {
>  	return NULL;
>  }
> -static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
> -{
> -}
>  #endif /* CONFIG_BLOCK && CONFIG_FS_DAX */
>  
>  #if IS_ENABLED(CONFIG_FS_DAX)
> +void fs_put_dax(struct dax_device *dax_dev, void *holder);
> +int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +	       const struct dax_holder_operations *hops);
> +struct dax_device *inode_dax(struct inode *inode);
>  int dax_writeback_mapping_range(struct address_space *mapping,
>  		struct dax_device *dax_dev, struct writeback_control *wbc);
>  int dax_folio_reset_order(struct folio *folio);
> @@ -164,6 +164,15 @@ dax_entry_t dax_lock_mapping_entry(struct address_space *mapping,
>  void dax_unlock_mapping_entry(struct address_space *mapping,
>  		unsigned long index, dax_entry_t cookie);
>  #else
> +static inline void fs_put_dax(struct dax_device *dax_dev, void *holder)
> +{
> +}
> +
> +static inline int fs_dax_get(struct dax_device *dax_dev, void *holder,
> +			     const struct dax_holder_operations *hops)
> +{
> +	return -EOPNOTSUPP;
> +}
>  static inline struct page *dax_layout_busy_page(struct address_space *mapping)
>  {
>  	return NULL;

[PATCH V9 8/8] dax: export dax_dev_get()

[John Groves]

From: John Groves <john@groves.net>

famfs needs to look up a dax_device by dev_t when resolving fmap
entries that reference character dax devices.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/super.c | 3 ++-
 include/linux/dax.h | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index d4ab60c406bf..25cf99dd9360 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -521,7 +521,7 @@ static int dax_set(struct inode *inode, void *data)
 	return 0;
 }
 
-static struct dax_device *dax_dev_get(dev_t devt)
+struct dax_device *dax_dev_get(dev_t devt)
 {
 	struct dax_device *dax_dev;
 	struct inode *inode;
@@ -544,6 +544,7 @@ static struct dax_device *dax_dev_get(dev_t devt)
 
 	return dax_dev;
 }
+EXPORT_SYMBOL_GPL(dax_dev_get);
 
 struct dax_device *alloc_dax(void *private, const struct dax_operations *ops)
 {
diff --git a/include/linux/dax.h b/include/linux/dax.h
index bf37b9a982f3..8c7e23ddf857 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -54,6 +54,7 @@ struct dax_device *alloc_dax(void *private, const struct dax_operations *ops);
 void *dax_holder(struct dax_device *dax_dev);
 void put_dax(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
+struct dax_device *dax_dev_get(dev_t devt);
 void dax_write_cache(struct dax_device *dax_dev, bool wc);
 bool dax_write_cache_enabled(struct dax_device *dax_dev);
 bool dax_synchronous(struct dax_device *dax_dev);
-- 
2.53.0

Re: [PATCH V9 8/8] dax: export dax_dev_get()

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:39:43 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <john@groves.net>
> 
> famfs needs to look up a dax_device by dev_t when resolving fmap
> entries that reference character dax devices.
> 
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: John Groves <john@groves.net>

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

[PATCH V9 00/10] famfs: port into fuse

[John Groves]

From: John Groves <john@groves.net>

This patch series on top of the prerequisite patches to dax are
available as a git tag at [0].

NOTE: this series depends on the famfs dax series
(dax: prepare for famfs)!!

Changes v8 -> v9
- Kconfig: fs/fuse/Kconfig:CONFIG_FUSE_FAMFS_DAX now depends on the
  new CONFIG_DEV_DAX_FSDEV (from drivers/dax/Kconfig) rather than
  just CONFIG_DEV_DAX and CONFIG_FS_DAX. (CONFIG_FUSE_FAMFS_DAX
  depends on those...)

Description:

This patch series introduces famfs into the fuse file system framework.
Famfs depends on the bundled dax patch set.

The famfs user space code can be found at [1].

Fuse Overview:

Famfs started as a standalone file system, but this series is intended to
permanently supersede that implementation. At a high level, famfs adds
two new fuse server messages:

GET_FMAP   - Retrieves a famfs fmap (the file-to-dax map for a famfs
	     file)
GET_DAXDEV - Retrieves the details of a particular daxdev that was
	     referenced by an fmap

Famfs Overview

Famfs exposes shared memory as a file system. Famfs consumes shared
memory from dax devices, and provides memory-mappable files that map
directly to the memory - no page cache involvement. Famfs differs from
conventional file systems in fs-dax mode, in that it handles in-memory
metadata in a sharable way (which begins with never caching dirty shared
metadata).

Famfs started as a standalone file system [2,3], but the consensus at
LSFMM was that it should be ported into fuse [4,5].

The key performance requirement is that famfs must resolve mapping faults
without upcalls. This is achieved by fully caching the file-to-devdax
metadata for all active files. This is done via two fuse client/server
message/response pairs: GET_FMAP and GET_DAXDEV.

Famfs remains the first fs-dax file system that is backed by devdax
rather than pmem in fs-dax mode (hence the need for the new dax mode).

Notes

- When a file is opened in a famfs mount, the OPEN is followed by a
  GET_FMAP message and response. The "fmap" is the full file-to-dax
  mapping, allowing the fuse/famfs kernel code to handle
  read/write/fault without any upcalls.

- After each GET_FMAP, the fmap is checked for extents that reference
  previously-unknown daxdevs. Each such occurrence is handled with a
  GET_DAXDEV message and response.

- Daxdevs are stored in a table (which might become an xarray at some
  point). When entries are added to the table, we acquire exclusive
  access to the daxdev via the fs_dax_get() call (modeled after how
  fs-dax handles this with pmem devices). Famfs provides
  holder_operations to devdax, providing a notification path in the
  event of memory errors or forced reconfiguration.

- If devdax notifies famfs of memory errors on a dax device, famfs
  currently blocks all subsequent accesses to data on that device. The
  recovery is to re-initialize the memory and file system. Famfs is
  memory, not storage...

- Because famfs uses backing (devdax) devices, only privileged mounts are
  supported (i.e. the fuse server requires CAP_SYS_RAWIO).

- The famfs kernel code never accesses the memory directly - it only
  facilitates read, write and mmap on behalf of user processes, using
  fmap metadata provided by its privileged fuse server. As such, the
  RAS of the shared memory affects applications, but not the kernel.

- Famfs has backing device(s), but they are devdax (char) rather than
  block. Right now there is no way to tell the vfs layer that famfs has a
  char backing device (unless we say it's block, but it's not). Currently
  we use the standard anonymous fuse fs_type - but I'm not sure that's
  ultimately optimal (thoughts?)

Changes v7 -> v8
- Moved to inline __free declaration in fuse_get_fmap() and
  famfs_fuse_meta_alloc(), famfs_teardown()
- Adopted FIELD_PREP() macro rather than manual bitfield manipulation
- Minor doc edits
- I dropped adding magic numbers to include/uapi/linux/magic.h. That
  can be done later if appropriate

Changes v6 -> v7
- Fixed a regression in famfs_interleave_fileofs_to_daxofs() that
  was reported by Intel's kernel test robot
- Added a check in __fsdev_dax_direct_access() for negative return
  from pgoff_to_phys(), which would indicate an out-of-range offset
- Fixed a bug in __famfs_meta_free(), where not all interleaved
  extents were freed
- Added chunksize alignment checks in famfs_fuse_meta_alloc() and
  famfs_interleave_fileofs_to_daxofs() as interleaved chunks must
  be PTE or PMD aligned
- Simplified famfs_file_init_dax() a bit
- Re-ran CM's kernel code review prompts on the entire series and
  fixed several minor issues

Changes v4 -> v5 -> v6
- None. Re-sending due to technical difficulties

Changes v3 [9] -> v4
- The patch "dax: prevent driver unbind while filesystem holds device"
  has been dropped. Dan Williams indicated that the favored behavior is
  for a file system to stop working if an underlying driver is unbound,
  rather than preventing the unbind.
- The patch "famfs_fuse: Famfs mount opt: -o shadow=<shadowpath>" has
  been dropped. Found a way for the famfs user space to do without the
  -o opt (via getxattr).
- Squashed the fs/fuse/Kconfig patch into the first subsequent patch
  that needed the change
  ("famfs_fuse: Basic fuse kernel ABI enablement for famfs")
- Many review comments addressed.
- Addressed minor kerneldoc infractions reported by test robot.

Changes v2 [7] -> v3
- Dax: Completely new fsdev driver (drivers/dax/fsdev.c) replaces the
  dev_dax_iomap modifications to bus.c/device.c. Devdax devices can now
  be switched among 'devdax', 'famfs' and 'system-ram' modes via daxctl
  or sysfs.
- Dax: fsdev uses MEMORY_DEVICE_FS_DAX type and leaves folios at order-0
  (no vmemmap_shift), allowing fs-dax to manage folio lifecycles
  dynamically like pmem does.
- Dax: The "poisoned page" problem is properly fixed via
  fsdev_clear_folio_state(), which clears stale mapping/compound state
  when fsdev binds. The temporary WARN_ON_ONCE workaround in fs/dax.c
  has been removed.
- Dax: Added dax_set_ops() so fsdev can set dax_operations at bind time
  (and clear them on unbind), since the dax_device is created before we
  know which driver will bind.
- Dax: Added custom bind/unbind sysfs handlers; unbind return -EBUSY if a
  filesystem holds the device, preventing unbind while famfs is mounted.
- Fuse: Famfs mounts now require that the fuse server/daemon has
  CAP_SYS_RAWIO because they expose raw memory devices.
- Fuse: Added DAX address_space_operations with noop_dirty_folio since
  famfs is memory-backed with no writeback required.
- Rebased to latest kernels, fully compatible with Alistair Popple
  et. al's recent dax refactoring.
- Ran this series through Chris Mason's code review AI prompts to check
  for issues - several subtle problems found and fixed.
- Dropped RFC status - this version is intended to be mergeable.

Changes v1 [8] -> v2:

- The GET_FMAP message/response has been moved from LOOKUP to OPEN, as
  was the pretty much unanimous consensus.
- Made the response payload to GET_FMAP variable sized (patch 12)
- Dodgy kerneldoc comments cleaned up or removed.
- Fixed memory leak of fc->shadow in patch 11 (thanks Joanne)
- Dropped many pr_debug and pr_notice calls


References

[0] - https://github.com/jagalactic/linux/tree/famfs-v9 (this patch set)
[1] - https://famfs.org (famfs user space)
[2] - https://lore.kernel.org/linux-cxl/cover.1708709155.git.john@groves.net/
[3] - https://lore.kernel.org/linux-cxl/cover.1714409084.git.john@groves.net/
[4] - https://lwn.net/Articles/983105/ (lsfmm 2024)
[5] - https://lwn.net/Articles/1020170/ (lsfmm 2025)
[6] - https://lore.kernel.org/linux-cxl/cover.8068ad144a7eea4a813670301f4d2a86a8e68ec4.1740713401.git-series.apopple@nvidia.com/
[7] - https://lore.kernel.org/linux-fsdevel/20250703185032.46568-1-john@groves.net/ (famfs fuse v2)
[8] - https://lore.kernel.org/linux-fsdevel/20250421013346.32530-1-john@groves.net/ (famfs fuse v1)
[9] - https://lore.kernel.org/linux-fsdevel/20260107153244.64703-1-john@groves.net/T/#mb2c868801be16eca82dab239a1d201628534aea7 (famfs fuse v3)


John Groves (10):
  famfs_fuse: Update macro s/FUSE_IS_DAX/FUSE_IS_VIRTIO_DAX/
  famfs_fuse: Basic fuse kernel ABI enablement for famfs
  famfs_fuse: Plumb the GET_FMAP message/response
  famfs_fuse: Create files with famfs fmaps
  famfs_fuse: GET_DAXDEV message and daxdev_table
  famfs_fuse: Plumb dax iomap and fuse read/write/mmap
  famfs_fuse: Add holder_operations for dax notify_failure()
  famfs_fuse: Add DAX address_space_operations with noop_dirty_folio
  famfs_fuse: Add famfs fmap metadata documentation
  famfs_fuse: Add documentation

 Documentation/filesystems/famfs.rst |  142 ++++
 Documentation/filesystems/index.rst |    1 +
 MAINTAINERS                         |   10 +
 fs/fuse/Kconfig                     |   13 +
 fs/fuse/Makefile                    |    1 +
 fs/fuse/dir.c                       |    2 +-
 fs/fuse/famfs.c                     | 1180 +++++++++++++++++++++++++++
 fs/fuse/famfs_kfmap.h               |  167 ++++
 fs/fuse/file.c                      |   45 +-
 fs/fuse/fuse_i.h                    |  116 ++-
 fs/fuse/inode.c                     |   35 +-
 fs/fuse/iomode.c                    |    2 +-
 fs/namei.c                          |    1 +
 include/uapi/linux/fuse.h           |   88 ++
 14 files changed, 1790 insertions(+), 13 deletions(-)
 create mode 100644 Documentation/filesystems/famfs.rst
 create mode 100644 fs/fuse/famfs.c
 create mode 100644 fs/fuse/famfs_kfmap.h


base-commit: 1a6b38b1a257ac755e31f518dc82cad472396a1d
-- 
2.53.0

[PATCH V9 01/10] famfs_fuse: Update macro s/FUSE_IS_DAX/FUSE_IS_VIRTIO_DAX/

[John Groves]

From: John Groves <john@groves.net>

Virtio_fs now needs to determine if an inode is DAX && not famfs.
This relaces the FUSE_IS_DAX() macro with FUSE_IS_VIRTIO_DAX(),
in preparation for famfs in later commits. The dummy
fuse_file_famfs() macro will be replaced with a working
function.

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/dir.c    |  2 +-
 fs/fuse/file.c   | 13 ++++++++-----
 fs/fuse/fuse_i.h |  9 ++++++++-
 fs/fuse/inode.c  |  4 ++--
 fs/fuse/iomode.c |  2 +-
 5 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 7ac6b232ef12..c63f097bc697 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -2161,7 +2161,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 		is_truncate = true;
 	}
 
-	if (FUSE_IS_DAX(inode) && is_truncate) {
+	if (FUSE_IS_VIRTIO_DAX(fi) && is_truncate) {
 		filemap_invalidate_lock(mapping);
 		fault_blocked = true;
 		err = fuse_dax_break_layouts(inode, 0, -1);
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b1bb7153cb78..4ee5065737d8 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -252,7 +252,7 @@ static int fuse_open(struct inode *inode, struct file *file)
 	int err;
 	bool is_truncate = (file->f_flags & O_TRUNC) && fc->atomic_o_trunc;
 	bool is_wb_truncate = is_truncate && fc->writeback_cache;
-	bool dax_truncate = is_truncate && FUSE_IS_DAX(inode);
+	bool dax_truncate = is_truncate && FUSE_IS_VIRTIO_DAX(fi);
 
 	if (fuse_is_bad(inode))
 		return -EIO;
@@ -1812,11 +1812,12 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	struct file *file = iocb->ki_filp;
 	struct fuse_file *ff = file->private_data;
 	struct inode *inode = file_inode(file);
+	struct fuse_inode *fi = get_fuse_inode(inode);
 
 	if (fuse_is_bad(inode))
 		return -EIO;
 
-	if (FUSE_IS_DAX(inode))
+	if (FUSE_IS_VIRTIO_DAX(fi))
 		return fuse_dax_read_iter(iocb, to);
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
@@ -1833,11 +1834,12 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct file *file = iocb->ki_filp;
 	struct fuse_file *ff = file->private_data;
 	struct inode *inode = file_inode(file);
+	struct fuse_inode *fi = get_fuse_inode(inode);
 
 	if (fuse_is_bad(inode))
 		return -EIO;
 
-	if (FUSE_IS_DAX(inode))
+	if (FUSE_IS_VIRTIO_DAX(fi))
 		return fuse_dax_write_iter(iocb, from);
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
@@ -2370,10 +2372,11 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
 	struct fuse_file *ff = file->private_data;
 	struct fuse_conn *fc = ff->fm->fc;
 	struct inode *inode = file_inode(file);
+	struct fuse_inode *fi = get_fuse_inode(inode);
 	int rc;
 
 	/* DAX mmap is superior to direct_io mmap */
-	if (FUSE_IS_DAX(inode))
+	if (FUSE_IS_VIRTIO_DAX(fi))
 		return fuse_dax_mmap(file, vma);
 
 	/*
@@ -2934,7 +2937,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 		.mode = mode
 	};
 	int err;
-	bool block_faults = FUSE_IS_DAX(inode) &&
+	bool block_faults = FUSE_IS_VIRTIO_DAX(fi) &&
 		(!(mode & FALLOC_FL_KEEP_SIZE) ||
 		 (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)));
 
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7f16049387d1..45e108dec771 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1508,7 +1508,14 @@ void fuse_free_conn(struct fuse_conn *fc);
 
 /* dax.c */
 
-#define FUSE_IS_DAX(inode) (IS_ENABLED(CONFIG_FUSE_DAX) && IS_DAX(inode))
+static inline bool fuse_file_famfs(struct fuse_inode *fuse_inode) /* Will be superseded */
+{
+	(void)fuse_inode;
+	return false;
+}
+#define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX)	\
+					&& IS_DAX(&fuse_inode->inode)  \
+					&& !fuse_file_famfs(fuse_inode))
 
 ssize_t fuse_dax_read_iter(struct kiocb *iocb, struct iov_iter *to);
 ssize_t fuse_dax_write_iter(struct kiocb *iocb, struct iov_iter *from);
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index e57b8af06be9..1333b3ebb18a 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -162,7 +162,7 @@ static void fuse_evict_inode(struct inode *inode)
 	/* Will write inode on close/munmap and in all other dirtiers */
 	WARN_ON(inode_state_read_once(inode) & I_DIRTY_INODE);
 
-	if (FUSE_IS_DAX(inode))
+	if (FUSE_IS_VIRTIO_DAX(fi))
 		dax_break_layout_final(inode);
 
 	truncate_inode_pages_final(&inode->i_data);
@@ -170,7 +170,7 @@ static void fuse_evict_inode(struct inode *inode)
 	if (inode->i_sb->s_flags & SB_ACTIVE) {
 		struct fuse_conn *fc = get_fuse_conn(inode);
 
-		if (FUSE_IS_DAX(inode))
+		if (FUSE_IS_VIRTIO_DAX(fi))
 			fuse_dax_inode_cleanup(inode);
 		if (fi->nlookup) {
 			fuse_queue_forget(fc, fi->forget, fi->nodeid,
diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
index 3728933188f3..31ee7f3304c6 100644
--- a/fs/fuse/iomode.c
+++ b/fs/fuse/iomode.c
@@ -203,7 +203,7 @@ int fuse_file_io_open(struct file *file, struct inode *inode)
 	 * io modes are not relevant with DAX and with server that does not
 	 * implement open.
 	 */
-	if (FUSE_IS_DAX(inode) || !ff->args)
+	if (FUSE_IS_VIRTIO_DAX(fi) || !ff->args)
 		return 0;
 
 	/*
-- 
2.53.0

Re: [PATCH V9 01/10] famfs_fuse: Update macro s/FUSE_IS_DAX/FUSE_IS_VIRTIO_DAX/

[Jonathan Cameron]

On Tue, 24 Mar 2026 00:40:36 +0000
John Groves <john@jagalactic.com> wrote:

> From: John Groves <john@groves.net>
> 
> Virtio_fs now needs to determine if an inode is DAX && not famfs.
FWIW patch bundles broke sashiko

https://sashiko.dev/#/message/0100019d1d46d094-cc0a4b79-3bd2-43e8-a08d-ab8cd21266a6-000000%40email.amazonses.com
It only reviewed the fuse part.  (I was looking to see what it had found I missed
in the DAX ones).
> This relaces the FUSE_IS_DAX() macro with FUSE_IS_VIRTIO_DAX(),

replaces (Sashiko spell checks ;)

> in preparation for famfs in later commits. The dummy
> fuse_file_famfs() macro will be replaced with a working
> function.
> 
> Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
> Reviewed-by: Dave Jiang <dave.jiang@intel.com>
> Signed-off-by: John Groves <john@groves.net>



> ---
>  fs/fuse/dir.c    |  2 +-
>  fs/fuse/file.c   | 13 ++++++++-----
>  fs/fuse/fuse_i.h |  9 ++++++++-
>  fs/fuse/inode.c  |  4 ++--
>  fs/fuse/iomode.c |  2 +-
>  5 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 7ac6b232ef12..c63f097bc697 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -2161,7 +2161,7 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
>  		is_truncate = true;
>  	}
>  
> -	if (FUSE_IS_DAX(inode) && is_truncate) {
> +	if (FUSE_IS_VIRTIO_DAX(fi) && is_truncate) {
>  		filemap_invalidate_lock(mapping);
>  		fault_blocked = true;
>  		err = fuse_dax_break_layouts(inode, 0, -1);
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index b1bb7153cb78..4ee5065737d8 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -252,7 +252,7 @@ static int fuse_open(struct inode *inode, struct file *file)
>  	int err;
>  	bool is_truncate = (file->f_flags & O_TRUNC) && fc->atomic_o_trunc;
>  	bool is_wb_truncate = is_truncate && fc->writeback_cache;
> -	bool dax_truncate = is_truncate && FUSE_IS_DAX(inode);
> +	bool dax_truncate = is_truncate && FUSE_IS_VIRTIO_DAX(fi);
>  
>  	if (fuse_is_bad(inode))
>  		return -EIO;
> @@ -1812,11 +1812,12 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  	struct file *file = iocb->ki_filp;
>  	struct fuse_file *ff = file->private_data;
>  	struct inode *inode = file_inode(file);
> +	struct fuse_inode *fi = get_fuse_inode(inode);
>  
>  	if (fuse_is_bad(inode))
>  		return -EIO;
>  
> -	if (FUSE_IS_DAX(inode))
> +	if (FUSE_IS_VIRTIO_DAX(fi))
>  		return fuse_dax_read_iter(iocb, to);
>  
>  	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
> @@ -1833,11 +1834,12 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	struct file *file = iocb->ki_filp;
>  	struct fuse_file *ff = file->private_data;
>  	struct inode *inode = file_inode(file);
> +	struct fuse_inode *fi = get_fuse_inode(inode);
>  
>  	if (fuse_is_bad(inode))
>  		return -EIO;
>  
> -	if (FUSE_IS_DAX(inode))
> +	if (FUSE_IS_VIRTIO_DAX(fi))
>  		return fuse_dax_write_iter(iocb, from);
>  
>  	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
> @@ -2370,10 +2372,11 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
>  	struct fuse_file *ff = file->private_data;
>  	struct fuse_conn *fc = ff->fm->fc;
>  	struct inode *inode = file_inode(file);
> +	struct fuse_inode *fi = get_fuse_inode(inode);
>  	int rc;
>  
>  	/* DAX mmap is superior to direct_io mmap */
> -	if (FUSE_IS_DAX(inode))
> +	if (FUSE_IS_VIRTIO_DAX(fi))
>  		return fuse_dax_mmap(file, vma);
>  
>  	/*
> @@ -2934,7 +2937,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
>  		.mode = mode
>  	};
>  	int err;
> -	bool block_faults = FUSE_IS_DAX(inode) &&
> +	bool block_faults = FUSE_IS_VIRTIO_DAX(fi) &&
>  		(!(mode & FALLOC_FL_KEEP_SIZE) ||
>  		 (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)));
>  
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index 7f16049387d1..45e108dec771 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -1508,7 +1508,14 @@ void fuse_free_conn(struct fuse_conn *fc);
>  
>  /* dax.c */
>  
> -#define FUSE_IS_DAX(inode) (IS_ENABLED(CONFIG_FUSE_DAX) && IS_DAX(inode))
> +static inline bool fuse_file_famfs(struct fuse_inode *fuse_inode) /* Will be superseded */
> +{
> +	(void)fuse_inode;
> +	return false;
> +}
> +#define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX)	\
> +					&& IS_DAX(&fuse_inode->inode)  \
> +					&& !fuse_file_famfs(fuse_inode))

The AI overlord pointed out you should probably have a few more brackets
just in case someone passes something odd in as fuse_inode. Lets assume
they don't pass things with side effects in.

#define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX)	\
					&& IS_DAX(&(fuse_inode)->inode)  \
					&& !fuse_file_famfs(fuse_inode))


>  
>  ssize_t fuse_dax_read_iter(struct kiocb *iocb, struct iov_iter *to);
>  ssize_t fuse_dax_write_iter(struct kiocb *iocb, struct iov_iter *from);
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index e57b8af06be9..1333b3ebb18a 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -162,7 +162,7 @@ static void fuse_evict_inode(struct inode *inode)
>  	/* Will write inode on close/munmap and in all other dirtiers */
>  	WARN_ON(inode_state_read_once(inode) & I_DIRTY_INODE);
>  
> -	if (FUSE_IS_DAX(inode))
> +	if (FUSE_IS_VIRTIO_DAX(fi))
>  		dax_break_layout_final(inode);
>  
>  	truncate_inode_pages_final(&inode->i_data);
> @@ -170,7 +170,7 @@ static void fuse_evict_inode(struct inode *inode)
>  	if (inode->i_sb->s_flags & SB_ACTIVE) {
>  		struct fuse_conn *fc = get_fuse_conn(inode);
>  
> -		if (FUSE_IS_DAX(inode))
> +		if (FUSE_IS_VIRTIO_DAX(fi))
>  			fuse_dax_inode_cleanup(inode);
>  		if (fi->nlookup) {
>  			fuse_queue_forget(fc, fi->forget, fi->nodeid,
> diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
> index 3728933188f3..31ee7f3304c6 100644
> --- a/fs/fuse/iomode.c
> +++ b/fs/fuse/iomode.c
> @@ -203,7 +203,7 @@ int fuse_file_io_open(struct file *file, struct inode *inode)
>  	 * io modes are not relevant with DAX and with server that does not
>  	 * implement open.
>  	 */
> -	if (FUSE_IS_DAX(inode) || !ff->args)
> +	if (FUSE_IS_VIRTIO_DAX(fi) || !ff->args)
>  		return 0;
>  
>  	/*

[PATCH V9 02/10] famfs_fuse: Basic fuse kernel ABI enablement for famfs

[John Groves]

From: John Groves <john@groves.net>

This patch starts the kernel ABI enablement of famfs in fuse.

- Kconfig: Add FUSE_FAMFS_DAX config parameter, to control
  compilation of famfs within fuse.
- FUSE_DAX_FMAP flag in INIT request/reply
- fuse_conn->famfs_iomap (enable famfs-mapped files) to denote a
  famfs-enabled connection

Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/Kconfig           | 13 +++++++++++++
 fs/fuse/fuse_i.h          |  3 +++
 fs/fuse/inode.c           |  6 ++++++
 include/uapi/linux/fuse.h |  5 +++++
 4 files changed, 27 insertions(+)

diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
index 3a4ae632c94a..17fe1f490cbd 100644
--- a/fs/fuse/Kconfig
+++ b/fs/fuse/Kconfig
@@ -76,3 +76,16 @@ config FUSE_IO_URING
 
 	  If you want to allow fuse server/client communication through io-uring,
 	  answer Y
+
+config FUSE_FAMFS_DAX
+	bool "FUSE support for fs-dax filesystems backed by devdax"
+	depends on FUSE_FS
+	depends on DEV_DAX_FSDEV
+	default FUSE_FS
+	help
+	  This enables the fabric-attached memory file system (famfs),
+	  which enables formatting devdax memory as a file system. Famfs
+	  is primarily intended for scale-out shared access to
+	  disaggregated memory.
+
+	  To enable famfs or other fuse/fs-dax file systems, answer Y
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 45e108dec771..2839efb219a9 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -921,6 +921,9 @@ struct fuse_conn {
 	/* Is synchronous FUSE_INIT allowed? */
 	unsigned int sync_init:1;
 
+	/* dev_dax_iomap support for famfs */
+	unsigned int famfs_iomap:1;
+
 	/* Use io_uring for communication */
 	unsigned int io_uring;
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 1333b3ebb18a..fa77add7d9f8 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1456,6 +1456,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 
 			if (flags & FUSE_REQUEST_TIMEOUT)
 				timeout = arg->request_timeout;
+
+			if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
+			    flags & FUSE_DAX_FMAP)
+				fc->famfs_iomap = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1517,6 +1521,8 @@ static struct fuse_init_args *fuse_new_init(struct fuse_mount *fm)
 		flags |= FUSE_SUBMOUNTS;
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		flags |= FUSE_PASSTHROUGH;
+	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
+		flags |= FUSE_DAX_FMAP;
 
 	/*
 	 * This is just an information flag for fuse server. No need to check
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index c13e1f9a2f12..25686f088e6a 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -240,6 +240,9 @@
  *  - add FUSE_COPY_FILE_RANGE_64
  *  - add struct fuse_copy_file_range_out
  *  - add FUSE_NOTIFY_PRUNE
+ *
+ *  7.46
+ *  - Add FUSE_DAX_FMAP capability - ability to handle in-kernel fsdax maps
  */
 
 #ifndef _LINUX_FUSE_H
@@ -448,6 +451,7 @@ struct fuse_file_lock {
  * FUSE_OVER_IO_URING: Indicate that client supports io-uring
  * FUSE_REQUEST_TIMEOUT: kernel supports timing out requests.
  *			 init_out.request_timeout contains the timeout (in secs)
+ * FUSE_DAX_FMAP: kernel supports dev_dax_iomap (aka famfs) fmaps
  */
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
@@ -495,6 +499,7 @@ struct fuse_file_lock {
 #define FUSE_ALLOW_IDMAP	(1ULL << 40)
 #define FUSE_OVER_IO_URING	(1ULL << 41)
 #define FUSE_REQUEST_TIMEOUT	(1ULL << 42)
+#define FUSE_DAX_FMAP		(1ULL << 43)
 
 /**
  * CUSE INIT request/reply flags
-- 
2.53.0

[PATCH V9 03/10] famfs_fuse: Plumb the GET_FMAP message/response

[John Groves]

From: John Groves <john@groves.net>

Upon completion of an OPEN, if we're in famfs-mode we do a GET_FMAP to
retrieve and cache up the file-to-dax map in the kernel. If this
succeeds, read/write/mmap are resolved direct-to-dax with no upcalls.

Signed-off-by: John Groves <john@groves.net>
---
 MAINTAINERS               |  8 +++++
 fs/fuse/Makefile          |  1 +
 fs/fuse/famfs.c           | 73 +++++++++++++++++++++++++++++++++++++++
 fs/fuse/file.c            | 14 +++++++-
 fs/fuse/fuse_i.h          | 70 ++++++++++++++++++++++++++++++++++---
 fs/fuse/inode.c           |  8 ++++-
 fs/fuse/iomode.c          |  2 +-
 include/uapi/linux/fuse.h |  7 ++++
 8 files changed, 175 insertions(+), 8 deletions(-)
 create mode 100644 fs/fuse/famfs.c

diff --git a/MAINTAINERS b/MAINTAINERS
index eedf4cce56ed..71040a6494a3 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10512,6 +10512,14 @@ F:	fs/fuse/
 F:	include/uapi/linux/fuse.h
 F:	tools/testing/selftests/filesystems/fuse/
 
+FUSE [FAMFS Fabric-Attached Memory File System]
+M:	John Groves <jgroves@micron.com>
+M:	John Groves <John@Groves.net>
+L:	linux-cxl@vger.kernel.org
+L:	linux-fsdevel@vger.kernel.org
+S:	Supported
+F:	fs/fuse/famfs.c
+
 FUTEX SUBSYSTEM
 M:	Thomas Gleixner <tglx@kernel.org>
 M:	Ingo Molnar <mingo@redhat.com>
diff --git a/fs/fuse/Makefile b/fs/fuse/Makefile
index 22ad9538dfc4..3f8dcc8cbbd0 100644
--- a/fs/fuse/Makefile
+++ b/fs/fuse/Makefile
@@ -17,5 +17,6 @@ fuse-$(CONFIG_FUSE_DAX) += dax.o
 fuse-$(CONFIG_FUSE_PASSTHROUGH) += passthrough.o backing.o
 fuse-$(CONFIG_SYSCTL) += sysctl.o
 fuse-$(CONFIG_FUSE_IO_URING) += dev_uring.o
+fuse-$(CONFIG_FUSE_FAMFS_DAX) += famfs.o
 
 virtiofs-y := virtio_fs.o
diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
new file mode 100644
index 000000000000..d238d853afa8
--- /dev/null
+++ b/fs/fuse/famfs.c
@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * famfs - dax file system for shared fabric-attached memory
+ *
+ * Copyright 2023-2026 Micron Technology, Inc.
+ *
+ * This file system, originally based on ramfs the dax support from xfs,
+ * is intended to allow multiple host systems to mount a common file system
+ * view of dax files that map to shared memory.
+ */
+
+#include <linux/cleanup.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/dax.h>
+#include <linux/iomap.h>
+#include <linux/path.h>
+#include <linux/namei.h>
+#include <linux/string.h>
+
+#include "fuse_i.h"
+
+
+#define FMAP_BUFSIZE PAGE_SIZE
+
+int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	size_t fmap_bufsize = FMAP_BUFSIZE;
+	u64 nodeid = get_node_id(inode);
+	ssize_t fmap_size;
+	int rc;
+
+	FUSE_ARGS(args);
+
+	/* Don't retrieve if we already have the famfs metadata */
+	if (fi->famfs_meta)
+		return 0;
+
+	void *fmap_buf __free(kfree) = kzalloc(FMAP_BUFSIZE, GFP_KERNEL);
+
+	if (!fmap_buf)
+		return -ENOMEM;
+
+	args.opcode = FUSE_GET_FMAP;
+	args.nodeid = nodeid;
+
+	/* Variable-sized output buffer
+	 * this causes fuse_simple_request() to return the size of the
+	 * output payload
+	 */
+	args.out_argvar = true;
+	args.out_numargs = 1;
+	args.out_args[0].size = fmap_bufsize;
+	args.out_args[0].value = fmap_buf;
+
+	/* Send GET_FMAP command */
+	rc = fuse_simple_request(fm, &args);
+	if (rc < 0) {
+		pr_err("%s: err=%d from fuse_simple_request()\n",
+		       __func__, rc);
+		return rc;
+	}
+	fmap_size = rc;
+
+	/* We retrieved the "fmap" (the file's map to memory), but
+	 * we haven't used it yet. A call to famfs_file_init_dax() will be added
+	 * here in a subsequent patch, when we add the ability to attach
+	 * fmaps to files.
+	 */
+
+	return 0;
+}
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 4ee5065737d8..d7fd934f4412 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -277,6 +277,16 @@ static int fuse_open(struct inode *inode, struct file *file)
 	err = fuse_do_open(fm, get_node_id(inode), file, false);
 	if (!err) {
 		ff = file->private_data;
+
+		if ((fm->fc->famfs_iomap) && (S_ISREG(inode->i_mode))) {
+			/* Get the famfs fmap - failure is fatal */
+			err = fuse_get_fmap(fm, inode);
+			if (err) {
+				fuse_sync_release(fi, ff, file->f_flags);
+				goto out_nowrite;
+			}
+		}
+
 		err = fuse_finish_open(inode, file);
 		if (err)
 			fuse_sync_release(fi, ff, file->f_flags);
@@ -284,12 +294,14 @@ static int fuse_open(struct inode *inode, struct file *file)
 			fuse_truncate_update_attr(inode, file);
 	}
 
+out_nowrite:
 	if (is_wb_truncate || dax_truncate)
 		fuse_release_nowrite(inode);
 	if (!err) {
 		if (is_truncate)
 			truncate_pagecache(inode, 0);
-		else if (!(ff->open_flags & FOPEN_KEEP_CACHE))
+		else if (!(ff->open_flags & FOPEN_KEEP_CACHE) &&
+			 !fuse_file_famfs(fi))
 			invalidate_inode_pages2(inode->i_mapping);
 	}
 	if (dax_truncate)
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 2839efb219a9..b66b5ca0bc11 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -223,6 +223,14 @@ struct fuse_inode {
 	 * so preserve the blocksize specified by the server.
 	 */
 	u8 cached_i_blkbits;
+
+#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+	/* Pointer to the file's famfs metadata. Primary content is the
+	 * in-memory version of the fmap - the map from file's offset range
+	 * to DAX memory
+	 */
+	void *famfs_meta;
+#endif
 };
 
 /** FUSE inode state bits */
@@ -1511,11 +1519,8 @@ void fuse_free_conn(struct fuse_conn *fc);
 
 /* dax.c */
 
-static inline bool fuse_file_famfs(struct fuse_inode *fuse_inode) /* Will be superseded */
-{
-	(void)fuse_inode;
-	return false;
-}
+static inline int fuse_file_famfs(struct fuse_inode *fi); /* forward */
+
 #define FUSE_IS_VIRTIO_DAX(fuse_inode) (IS_ENABLED(CONFIG_FUSE_DAX)	\
 					&& IS_DAX(&fuse_inode->inode)  \
 					&& !fuse_file_famfs(fuse_inode))
@@ -1634,4 +1639,59 @@ extern void fuse_sysctl_unregister(void);
 #define fuse_sysctl_unregister()	do { } while (0)
 #endif /* CONFIG_SYSCTL */
 
+/* famfs.c */
+
+#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+void __famfs_meta_free(void *map);
+
+/* Set fi->famfs_meta = NULL regardless of prior value */
+static inline void famfs_meta_init(struct fuse_inode *fi)
+{
+	fi->famfs_meta = NULL;
+}
+
+/* Set fi->famfs_meta iff the current value is NULL */
+static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
+						  void *meta)
+{
+	return cmpxchg(&fi->famfs_meta, NULL, meta);
+}
+
+static inline void famfs_meta_free(struct fuse_inode *fi)
+{
+	famfs_meta_set(fi, NULL);
+}
+
+static inline int fuse_file_famfs(struct fuse_inode *fi)
+{
+	return (READ_ONCE(fi->famfs_meta) != NULL);
+}
+
+int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
+
+#else /* !CONFIG_FUSE_FAMFS_DAX */
+
+static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
+						  void *meta)
+{
+	return NULL;
+}
+
+static inline void famfs_meta_free(struct fuse_inode *fi)
+{
+}
+
+static inline int fuse_file_famfs(struct fuse_inode *fi)
+{
+	return 0;
+}
+
+static inline int
+fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
+{
+	return 0;
+}
+
+#endif /* CONFIG_FUSE_FAMFS_DAX */
+
 #endif /* _FS_FUSE_I_H */
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index fa77add7d9f8..e39a00c79085 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -120,6 +120,9 @@ static struct inode *fuse_alloc_inode(struct super_block *sb)
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		fuse_inode_backing_set(fi, NULL);
 
+	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
+		famfs_meta_set(fi, NULL);
+
 	return &fi->inode;
 
 out_free_forget:
@@ -141,6 +144,9 @@ static void fuse_free_inode(struct inode *inode)
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		fuse_backing_put(fuse_inode_backing(fi));
 
+	if (S_ISREG(inode->i_mode) && fuse_file_famfs(fi))
+		famfs_meta_free(fi);
+
 	kmem_cache_free(fuse_inode_cachep, fi);
 }
 
@@ -162,7 +168,7 @@ static void fuse_evict_inode(struct inode *inode)
 	/* Will write inode on close/munmap and in all other dirtiers */
 	WARN_ON(inode_state_read_once(inode) & I_DIRTY_INODE);
 
-	if (FUSE_IS_VIRTIO_DAX(fi))
+	if (FUSE_IS_VIRTIO_DAX(fi) || fuse_file_famfs(fi))
 		dax_break_layout_final(inode);
 
 	truncate_inode_pages_final(&inode->i_data);
diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c
index 31ee7f3304c6..948148316ef0 100644
--- a/fs/fuse/iomode.c
+++ b/fs/fuse/iomode.c
@@ -203,7 +203,7 @@ int fuse_file_io_open(struct file *file, struct inode *inode)
 	 * io modes are not relevant with DAX and with server that does not
 	 * implement open.
 	 */
-	if (FUSE_IS_VIRTIO_DAX(fi) || !ff->args)
+	if (FUSE_IS_VIRTIO_DAX(fi) || fuse_file_famfs(fi) || !ff->args)
 		return 0;
 
 	/*
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 25686f088e6a..9eff9083d3b5 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -669,6 +669,9 @@ enum fuse_opcode {
 	FUSE_STATX		= 52,
 	FUSE_COPY_FILE_RANGE_64	= 53,
 
+	/* Famfs / devdax opcodes */
+	FUSE_GET_FMAP           = 54,
+
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
 
@@ -1313,4 +1316,8 @@ struct fuse_uring_cmd_req {
 	uint8_t padding[6];
 };
 
+/* Famfs fmap message components */
+
+#define FAMFS_FMAP_MAX 32768 /* Largest supported fmap message */
+
 #endif /* _LINUX_FUSE_H */
-- 
2.53.0

[PATCH V9 04/10] famfs_fuse: Create files with famfs fmaps

[John Groves]

From: John Groves <john@groves.net>

On completion of GET_FMAP message/response, setup the full famfs
metadata such that it's possible to handle read/write/mmap directly to
dax. Note that the devdax_iomap plumbing is not in yet...

* Add famfs_kfmap.h: in-memory structures for resolving famfs file maps
  (fmaps) to dax.
* famfs.c: allocate, initialize and free fmaps
* inode.c: only allow famfs mode if the fuse server has CAP_SYS_RAWIO
* Update MAINTAINERS for the new file.

Signed-off-by: John Groves <john@groves.net>
---
 MAINTAINERS               |   1 +
 fs/fuse/famfs.c           | 339 +++++++++++++++++++++++++++++++++++++-
 fs/fuse/famfs_kfmap.h     |  67 ++++++++
 fs/fuse/fuse_i.h          |   8 +-
 fs/fuse/inode.c           |  20 ++-
 include/uapi/linux/fuse.h |  56 +++++++
 6 files changed, 481 insertions(+), 10 deletions(-)
 create mode 100644 fs/fuse/famfs_kfmap.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 71040a6494a3..c590988881f6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10519,6 +10519,7 @@ L:	linux-cxl@vger.kernel.org
 L:	linux-fsdevel@vger.kernel.org
 S:	Supported
 F:	fs/fuse/famfs.c
+F:	fs/fuse/famfs_kfmap.h
 
 FUTEX SUBSYSTEM
 M:	Thomas Gleixner <tglx@kernel.org>
diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index d238d853afa8..ac52e54e2cb5 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -18,9 +18,339 @@
 #include <linux/namei.h>
 #include <linux/string.h>
 
+#include "famfs_kfmap.h"
 #include "fuse_i.h"
 
 
+/***************************************************************************/
+
+void __famfs_meta_free(void *famfs_meta)
+{
+	struct famfs_file_meta *fmap = famfs_meta;
+
+	if (!fmap)
+		return;
+
+	switch (fmap->fm_extent_type) {
+	case SIMPLE_DAX_EXTENT:
+		kfree(fmap->se);
+		break;
+	case INTERLEAVED_EXTENT:
+		if (fmap->ie) {
+			for (int i = 0; i < fmap->fm_niext; i++)
+				kfree(fmap->ie[i].ie_strips);
+		}
+		kfree(fmap->ie);
+		break;
+	default:
+		pr_err("%s: invalid fmap type\n", __func__);
+		break;
+	}
+
+	kfree(fmap);
+}
+DEFINE_FREE(__famfs_meta_free, void *, if (_T) __famfs_meta_free(_T))
+
+static int
+famfs_check_ext_alignment(struct famfs_meta_simple_ext *se)
+{
+	int errs = 0;
+
+	if (se->dev_index != 0)
+		errs++;
+
+	/* TODO: pass in alignment so we can support the other page sizes */
+	if (!IS_ALIGNED(se->ext_offset, PMD_SIZE))
+		errs++;
+
+	if (!IS_ALIGNED(se->ext_len, PMD_SIZE))
+		errs++;
+
+	return errs;
+}
+
+/**
+ * famfs_fuse_meta_alloc() - Allocate famfs file metadata
+ * @fmap_buf:  fmap buffer from fuse server
+ * @fmap_buf_size: size of fmap buffer
+ * @metap:         pointer where 'struct famfs_file_meta' is returned
+ *
+ * Returns: 0=success
+ *          -errno=failure
+ */
+static int
+famfs_fuse_meta_alloc(
+	void *fmap_buf,
+	size_t fmap_buf_size,
+	struct famfs_file_meta **metap)
+{
+	struct fuse_famfs_fmap_header *fmh;
+	size_t extent_total = 0;
+	size_t next_offset = 0;
+	int errs = 0;
+	int i, j;
+
+	fmh = fmap_buf;
+
+	/* Move past fmh in fmap_buf */
+	next_offset += sizeof(*fmh);
+	if (next_offset > fmap_buf_size) {
+		pr_err("%s:%d: fmap_buf underflow offset/size %ld/%ld\n",
+		       __func__, __LINE__, next_offset, fmap_buf_size);
+		return -EINVAL;
+	}
+
+	if (fmh->nextents < 1) {
+		pr_err("%s: nextents %d < 1\n", __func__, fmh->nextents);
+		return -ERANGE;
+	}
+
+	if (fmh->nextents > FUSE_FAMFS_MAX_EXTENTS) {
+		pr_err("%s: nextents %d > max (%d) 1\n",
+		       __func__, fmh->nextents, FUSE_FAMFS_MAX_EXTENTS);
+		return -ERANGE;
+	}
+
+	struct famfs_file_meta *meta __free(__famfs_meta_free) = kzalloc(sizeof(*meta), GFP_KERNEL);
+
+	if (!meta)
+		return -ENOMEM;
+
+	meta->error = false;
+	meta->file_type = fmh->file_type;
+	meta->file_size = fmh->file_size;
+	meta->fm_extent_type = fmh->ext_type;
+
+	switch (fmh->ext_type) {
+	case FUSE_FAMFS_EXT_SIMPLE: {
+		struct fuse_famfs_simple_ext *se_in;
+
+		se_in = fmap_buf + next_offset;
+
+		/* Move past simple extents */
+		next_offset += fmh->nextents * sizeof(*se_in);
+		if (next_offset > fmap_buf_size) {
+			pr_err("%s:%d: fmap_buf underflow offset/size %ld/%ld\n",
+			       __func__, __LINE__, next_offset, fmap_buf_size);
+			return -EINVAL;
+		}
+
+		meta->fm_nextents = fmh->nextents;
+
+		meta->se = kcalloc(meta->fm_nextents, sizeof(*(meta->se)),
+				   GFP_KERNEL);
+		if (!meta->se)
+			return -ENOMEM;
+
+		if ((meta->fm_nextents > FUSE_FAMFS_MAX_EXTENTS) ||
+		    (meta->fm_nextents < 1))
+			return -EINVAL;
+
+		for (i = 0; i < fmh->nextents; i++) {
+			meta->se[i].dev_index  = se_in[i].se_devindex;
+			meta->se[i].ext_offset = se_in[i].se_offset;
+			meta->se[i].ext_len    = se_in[i].se_len;
+
+			/* Record bitmap of referenced daxdev indices */
+			meta->dev_bitmap |= (1 << meta->se[i].dev_index);
+
+			errs += famfs_check_ext_alignment(&meta->se[i]);
+
+			extent_total += meta->se[i].ext_len;
+		}
+		break;
+	}
+
+	case FUSE_FAMFS_EXT_INTERLEAVE: {
+		s64 size_remainder = meta->file_size;
+		struct fuse_famfs_iext *ie_in;
+		int niext = fmh->nextents;
+
+		meta->fm_niext = niext;
+
+		/* Allocate interleaved extent */
+		meta->ie = kcalloc(niext, sizeof(*(meta->ie)), GFP_KERNEL);
+		if (!meta->ie)
+			return -ENOMEM;
+
+		/*
+		 * Each interleaved extent has a simple extent list of strips.
+		 * Outer loop is over separate interleaved extents
+		 */
+		for (i = 0; i < niext; i++) {
+			u64 nstrips;
+			struct fuse_famfs_simple_ext *sie_in;
+
+			/* ie_in = one interleaved extent in fmap_buf */
+			ie_in = fmap_buf + next_offset;
+
+			/* Move past one interleaved extent header in fmap_buf */
+			next_offset += sizeof(*ie_in);
+			if (next_offset > fmap_buf_size) {
+				pr_err("%s:%d: fmap_buf underflow offset/size %ld/%ld\n",
+				       __func__, __LINE__, next_offset,
+				       fmap_buf_size);
+				return -EINVAL;
+			}
+
+			if (!IS_ALIGNED(ie_in->ie_chunk_size, PMD_SIZE)) {
+				pr_err("%s: chunk_size %lld not PMD-aligned\n",
+				       __func__, meta->ie[i].fie_chunk_size);
+				return -EINVAL;
+			}
+
+			if (ie_in->ie_nbytes == 0) {
+				pr_err("%s: zero-length interleave!\n",
+				       __func__);
+				return -EINVAL;
+			}
+
+			nstrips = ie_in->ie_nstrips;
+			meta->ie[i].fie_chunk_size = ie_in->ie_chunk_size;
+			meta->ie[i].fie_nstrips    = ie_in->ie_nstrips;
+			meta->ie[i].fie_nbytes     = ie_in->ie_nbytes;
+
+			/* sie_in = the strip extents in fmap_buf */
+			sie_in = fmap_buf + next_offset;
+
+			/* Move past strip extents in fmap_buf */
+			next_offset += nstrips * sizeof(*sie_in);
+			if (next_offset > fmap_buf_size) {
+				pr_err("%s:%d: fmap_buf underflow offset/size %ld/%ld\n",
+				       __func__, __LINE__, next_offset,
+				       fmap_buf_size);
+				return -EINVAL;
+			}
+
+			if ((nstrips > FUSE_FAMFS_MAX_STRIPS) || (nstrips < 1)) {
+				pr_err("%s: invalid nstrips=%lld (max=%d)\n",
+				       __func__, nstrips,
+				       FUSE_FAMFS_MAX_STRIPS);
+				errs++;
+			}
+
+			/* Allocate strip extent array */
+			meta->ie[i].ie_strips =
+				kcalloc(ie_in->ie_nstrips,
+					sizeof(meta->ie[i].ie_strips[0]),
+					GFP_KERNEL);
+			if (!meta->ie[i].ie_strips)
+				return -ENOMEM;
+
+			/* Inner loop is over strips */
+			for (j = 0; j < nstrips; j++) {
+				struct famfs_meta_simple_ext *strips_out;
+				u64 devindex = sie_in[j].se_devindex;
+				u64 offset   = sie_in[j].se_offset;
+				u64 len      = sie_in[j].se_len;
+
+				strips_out = meta->ie[i].ie_strips;
+				strips_out[j].dev_index  = devindex;
+				strips_out[j].ext_offset = offset;
+				strips_out[j].ext_len    = len;
+
+				/* Record bitmap of referenced daxdev indices */
+				meta->dev_bitmap |= (1 << devindex);
+
+				extent_total += len;
+				errs += famfs_check_ext_alignment(&strips_out[j]);
+				size_remainder -= len;
+			}
+		}
+
+		if (size_remainder > 0) {
+			/* Sum of interleaved extent sizes is less than file size! */
+			pr_err("%s: size_remainder %lld (0x%llx)\n",
+			       __func__, size_remainder, size_remainder);
+			return -EINVAL;
+		}
+		break;
+	}
+
+	default:
+		pr_err("%s: invalid ext_type %d\n", __func__, fmh->ext_type);
+		return -EINVAL;
+	}
+
+	if (errs > 0) {
+		pr_err("%s: %d alignment errors found\n", __func__, errs);
+		return -EINVAL;
+	}
+
+	/* More sanity checks */
+	if (extent_total < meta->file_size) {
+		pr_err("%s: file size %ld larger than map size %ld\n",
+		       __func__, meta->file_size, extent_total);
+		return -EINVAL;
+	}
+
+	if (cmpxchg(metap, NULL, meta) != NULL) {
+		pr_debug("%s: fmap race detected\n", __func__);
+		return 0; /* fmap already installed */
+	}
+	retain_and_null_ptr(meta);
+
+	return 0;
+}
+
+/**
+ * famfs_file_init_dax() - init famfs dax file metadata
+ *
+ * @fm:        fuse_mount
+ * @inode:     the inode
+ * @fmap_buf:  fmap response message
+ * @fmap_size: Size of the fmap message
+ *
+ * Initialize famfs metadata for a file, based on the contents of the GET_FMAP
+ * response
+ *
+ * Return: 0=success
+ *          -errno=failure
+ */
+int
+famfs_file_init_dax(
+	struct fuse_mount *fm,
+	struct inode *inode,
+	void *fmap_buf,
+	size_t fmap_size)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct famfs_file_meta *meta = NULL;
+	int rc;
+
+	if (fi->famfs_meta) {
+		pr_notice("%s: i_no=%ld fmap_size=%ld ALREADY INITIALIZED\n",
+			  __func__,
+			  inode->i_ino, fmap_size);
+		return 0;
+	}
+
+	rc = famfs_fuse_meta_alloc(fmap_buf, fmap_size, &meta);
+	if (rc)
+		goto errout;
+
+	/* Publish the famfs metadata on fi->famfs_meta */
+	inode_lock(inode);
+
+	if (famfs_meta_set(fi, meta) == NULL) {
+		i_size_write(inode, meta->file_size);
+		inode->i_flags |= S_DAX;
+	} else {
+		pr_debug("%s: file already had metadata\n", __func__);
+		__famfs_meta_free(meta);
+		/* rc is 0 - the file is valid */
+	}
+
+	inode_unlock(inode);
+	return 0;
+
+errout:
+	if (rc)
+		__famfs_meta_free(meta);
+
+	return rc;
+}
+
 #define FMAP_BUFSIZE PAGE_SIZE
 
 int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
@@ -63,11 +393,8 @@ int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
 	}
 	fmap_size = rc;
 
-	/* We retrieved the "fmap" (the file's map to memory), but
-	 * we haven't used it yet. A call to famfs_file_init_dax() will be added
-	 * here in a subsequent patch, when we add the ability to attach
-	 * fmaps to files.
-	 */
+	/* Convert fmap into in-memory format and hang from inode */
+	rc = famfs_file_init_dax(fm, inode, fmap_buf, fmap_size);
 
-	return 0;
+	return rc;
 }
diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
new file mode 100644
index 000000000000..18ab22bcc5a1
--- /dev/null
+++ b/fs/fuse/famfs_kfmap.h
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * famfs - dax file system for shared fabric-attached memory
+ *
+ * Copyright 2023-2026 Micron Technology, Inc.
+ */
+#ifndef FAMFS_KFMAP_H
+#define FAMFS_KFMAP_H
+
+/*
+ * The structures below are the in-memory metadata format for famfs files.
+ * Metadata retrieved via the GET_FMAP response is converted to this format
+ * for use in resolving file mapping faults.
+ *
+ * The GET_FMAP response contains the same information, but in a more
+ * message-and-versioning-friendly format. Those structs can be found in the
+ * famfs section of include/uapi/linux/fuse.h (aka fuse_kernel.h in libfuse)
+ */
+
+enum famfs_file_type {
+	FAMFS_REG,
+	FAMFS_SUPERBLOCK,
+	FAMFS_LOG,
+};
+
+/* We anticipate the possibility of supporting additional types of extents */
+enum famfs_extent_type {
+	SIMPLE_DAX_EXTENT,
+	INTERLEAVED_EXTENT,
+	INVALID_EXTENT_TYPE,
+};
+
+struct famfs_meta_simple_ext {
+	u64 dev_index;
+	u64 ext_offset;
+	u64 ext_len;
+};
+
+struct famfs_meta_interleaved_ext {
+	u64 fie_nstrips;
+	u64 fie_chunk_size;
+	u64 fie_nbytes;
+	struct famfs_meta_simple_ext *ie_strips;
+};
+
+/*
+ * Each famfs dax file has this hanging from its fuse_inode->famfs_meta
+ */
+struct famfs_file_meta {
+	bool                   error;
+	enum famfs_file_type   file_type;
+	size_t                 file_size;
+	enum famfs_extent_type fm_extent_type;
+	u64 dev_bitmap; /* bitmap of referenced daxdevs by index */
+	union {
+		struct {
+			size_t         fm_nextents;
+			struct famfs_meta_simple_ext  *se;
+		};
+		struct {
+			size_t         fm_niext;
+			struct famfs_meta_interleaved_ext *ie;
+		};
+	};
+};
+
+#endif /* FAMFS_KFMAP_H */
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index b66b5ca0bc11..dbfec5b9c6e1 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1642,6 +1642,9 @@ extern void fuse_sysctl_unregister(void);
 /* famfs.c */
 
 #if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+int famfs_file_init_dax(struct fuse_mount *fm,
+			struct inode *inode, void *fmap_buf,
+			size_t fmap_size);
 void __famfs_meta_free(void *map);
 
 /* Set fi->famfs_meta = NULL regardless of prior value */
@@ -1659,7 +1662,10 @@ static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
 
 static inline void famfs_meta_free(struct fuse_inode *fi)
 {
-	famfs_meta_set(fi, NULL);
+	if (fi->famfs_meta != NULL) {
+		__famfs_meta_free(fi->famfs_meta);
+		famfs_meta_set(fi, NULL);
+	}
 }
 
 static inline int fuse_file_famfs(struct fuse_inode *fi)
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index e39a00c79085..d0c6037ca46b 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -10,6 +10,7 @@
 #include "fuse_dev_i.h"
 #include "dev_uring_i.h"
 
+#include <linux/bitfield.h>
 #include <linux/dax.h>
 #include <linux/pagemap.h>
 #include <linux/slab.h>
@@ -1464,8 +1465,21 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 				timeout = arg->request_timeout;
 
 			if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) &&
-			    flags & FUSE_DAX_FMAP)
-				fc->famfs_iomap = 1;
+			    flags & FUSE_DAX_FMAP) {
+				/* famfs_iomap is only allowed if the fuse
+				 * server has CAP_SYS_RAWIO. This was checked
+				 * in fuse_send_init, and FUSE_DAX_IOMAP was
+				 * set in in_flags if so. Only allow enablement
+				 * if we find it there. This function is
+				 * normally not running in fuse server context,
+				 * so we can't do the capability check here...
+				 */
+				u64 in_flags = FIELD_PREP(GENMASK_ULL(63, 32), ia->in.flags2)
+						| ia->in.flags;
+
+				if (in_flags & FUSE_DAX_FMAP)
+					fc->famfs_iomap = 1;
+			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
 			fc->no_lock = 1;
@@ -1527,7 +1541,7 @@ static struct fuse_init_args *fuse_new_init(struct fuse_mount *fm)
 		flags |= FUSE_SUBMOUNTS;
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		flags |= FUSE_PASSTHROUGH;
-	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
+	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX) && capable(CAP_SYS_RAWIO))
 		flags |= FUSE_DAX_FMAP;
 
 	/*
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index 9eff9083d3b5..cf678bebbfe0 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -243,6 +243,13 @@
  *
  *  7.46
  *  - Add FUSE_DAX_FMAP capability - ability to handle in-kernel fsdax maps
+ *  - Add the following structures for the GET_FMAP message reply components:
+ *    - struct fuse_famfs_simple_ext
+ *    - struct fuse_famfs_iext
+ *    - struct fuse_famfs_fmap_header
+ *  - Add the following enumerated types
+ *    - enum fuse_famfs_file_type
+ *    - enum famfs_ext_type
  */
 
 #ifndef _LINUX_FUSE_H
@@ -1318,6 +1325,55 @@ struct fuse_uring_cmd_req {
 
 /* Famfs fmap message components */
 
+#define FAMFS_FMAP_VERSION 1
+
 #define FAMFS_FMAP_MAX 32768 /* Largest supported fmap message */
+#define FUSE_FAMFS_MAX_EXTENTS 32
+#define FUSE_FAMFS_MAX_STRIPS 32
+
+enum fuse_famfs_file_type {
+	FUSE_FAMFS_FILE_REG,
+	FUSE_FAMFS_FILE_SUPERBLOCK,
+	FUSE_FAMFS_FILE_LOG,
+};
+
+enum famfs_ext_type {
+	FUSE_FAMFS_EXT_SIMPLE = 0,
+	FUSE_FAMFS_EXT_INTERLEAVE = 1,
+};
+
+struct fuse_famfs_simple_ext {
+	uint32_t se_devindex;
+	uint32_t reserved;
+	uint64_t se_offset;
+	uint64_t se_len;
+};
+
+struct fuse_famfs_iext { /* Interleaved extent */
+	uint32_t ie_nstrips;
+	uint32_t ie_chunk_size;
+	uint64_t ie_nbytes; /* Total bytes for this interleaved_ext;
+			     * sum of strips may be more
+			     */
+	uint64_t reserved;
+};
+
+struct fuse_famfs_fmap_header {
+	uint8_t file_type; /* enum famfs_file_type */
+	uint8_t reserved;
+	uint16_t fmap_version;
+	uint32_t ext_type; /* enum famfs_log_ext_type */
+	uint32_t nextents;
+	uint32_t reserved0;
+	uint64_t file_size;
+	uint64_t reserved1;
+};
+
+static inline int32_t fmap_msg_min_size(void)
+{
+	/* Smallest fmap message is a header plus one simple extent */
+	return (sizeof(struct fuse_famfs_fmap_header)
+		+ sizeof(struct fuse_famfs_simple_ext));
+}
 
 #endif /* _LINUX_FUSE_H */
-- 
2.53.0

[PATCH V9 05/10] famfs_fuse: GET_DAXDEV message and daxdev_table

[John Groves]

From: John Groves <john@groves.net>

- The new GET_DAXDEV message/response is added
- The famfs.c:famfs_teardown() function is added as a primary teardown
  function for famfs.
- The command it triggered by the update_daxdev_table() call, if there
  are any daxdevs in the subject fmap that are not represented in the
  daxdev_table yet.
- fs/namei.c: export may_open_dev()

Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/famfs.c           | 227 +++++++++++++++++++++++++++++++++++++-
 fs/fuse/famfs_kfmap.h     |  26 +++++
 fs/fuse/fuse_i.h          |  19 ++++
 fs/fuse/inode.c           |   7 +-
 fs/namei.c                |   1 +
 include/uapi/linux/fuse.h |  20 ++++
 6 files changed, 298 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index ac52e54e2cb5..0e9415aa6339 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -21,6 +21,228 @@
 #include "famfs_kfmap.h"
 #include "fuse_i.h"
 
+/*
+ * famfs_teardown()
+ *
+ * Deallocate famfs metadata for a fuse_conn
+ */
+void
+famfs_teardown(struct fuse_conn *fc)
+{
+	struct famfs_dax_devlist *devlist __free(kfree) = fc->dax_devlist;
+	int i;
+
+	fc->dax_devlist = NULL;
+
+	if (!devlist)
+		return;
+
+	if (!devlist->devlist)
+		return;
+
+	/* Close & release all the daxdevs in our table */
+	for (i = 0; i < devlist->nslots; i++) {
+		struct famfs_daxdev *dd = &devlist->devlist[i];
+
+		if (!dd->valid)
+			continue;
+
+		/* Release reference from dax_dev_get() */
+		if (dd->devp)
+			put_dax(dd->devp);
+
+		kfree(dd->name);
+	}
+	kfree(devlist->devlist);
+}
+
+static int
+famfs_verify_daxdev(const char *pathname, dev_t *devno)
+{
+	struct inode *inode;
+	struct path path;
+	int err;
+
+	if (!pathname || !*pathname)
+		return -EINVAL;
+
+	err = kern_path(pathname, LOOKUP_FOLLOW, &path);
+	if (err)
+		return err;
+
+	inode = d_backing_inode(path.dentry);
+	if (!S_ISCHR(inode->i_mode)) {
+		err = -EINVAL;
+		goto out_path_put;
+	}
+
+	if (!may_open_dev(&path)) { /* had to export this */
+		err = -EACCES;
+		goto out_path_put;
+	}
+
+	*devno = inode->i_rdev;
+
+out_path_put:
+	path_put(&path);
+	return err;
+}
+
+/**
+ * famfs_fuse_get_daxdev() - Retrieve info for a DAX device from fuse server
+ *
+ * Send a GET_DAXDEV message to the fuse server to retrieve info on a
+ * dax device.
+ *
+ * @fm:     fuse_mount
+ * @index:  the index of the dax device; daxdevs are referred to by index
+ *          in fmaps, and the server resolves the index to a particular daxdev
+ *
+ * Returns: 0=success
+ *          -errno=failure
+ */
+static int
+famfs_fuse_get_daxdev(struct fuse_mount *fm, const u64 index)
+{
+	struct fuse_daxdev_out daxdev_out = { 0 };
+	struct fuse_conn *fc = fm->fc;
+	struct famfs_daxdev *daxdev;
+	int rc;
+
+	FUSE_ARGS(args);
+
+	/* Store the daxdev in our table */
+	if (index >= fc->dax_devlist->nslots) {
+		pr_err("%s: index(%lld) > nslots(%d)\n",
+		       __func__, index, fc->dax_devlist->nslots);
+		return -EINVAL;
+	}
+
+	args.opcode = FUSE_GET_DAXDEV;
+	args.nodeid = index;
+
+	args.in_numargs = 0;
+
+	args.out_numargs = 1;
+	args.out_args[0].size = sizeof(daxdev_out);
+	args.out_args[0].value = &daxdev_out;
+
+	/* Send GET_DAXDEV command */
+	rc = fuse_simple_request(fm, &args);
+	if (rc) {
+		pr_err("%s: rc=%d from fuse_simple_request()\n",
+		       __func__, rc);
+		/* Error will be that the payload is smaller than FMAP_BUFSIZE,
+		 * which is the max we can handle. Empty payload handled below.
+		 */
+		return rc;
+	}
+
+	scoped_guard(rwsem_write, &fc->famfs_devlist_sem) {
+		daxdev = &fc->dax_devlist->devlist[index];
+
+		/* Abort if daxdev is now valid (races are possible here) */
+		if (daxdev->valid) {
+			pr_debug("%s: daxdev already known\n", __func__);
+			return 0;
+		}
+
+		/* Verify dev is valid and can be opened and gets the devno */
+		rc = famfs_verify_daxdev(daxdev_out.name, &daxdev->devno);
+		if (rc) {
+			pr_err("%s: rc=%d from famfs_verify_daxdev()\n",
+			       __func__, rc);
+			return rc;
+		}
+
+		daxdev->name = kstrdup(daxdev_out.name, GFP_KERNEL);
+		if (!daxdev->name)
+			return -ENOMEM;
+
+		/* This will fail if it's not a dax device */
+		daxdev->devp = dax_dev_get(daxdev->devno);
+		if (!daxdev->devp) {
+			pr_warn("%s: device %s not found or not dax\n",
+				__func__, daxdev_out.name);
+			kfree(daxdev->name);
+			daxdev->name = NULL;
+			return -ENODEV;
+		}
+
+		wmb(); /* All other fields must be visible before valid */
+		daxdev->valid = 1;
+	}
+
+	return 0;
+}
+
+/**
+ * famfs_update_daxdev_table() - Update the daxdev table
+ * @fm:   fuse_mount
+ * @meta: famfs_file_meta, in-memory format, built from a GET_FMAP response
+ *
+ * This function is called for each new file fmap, to verify whether all
+ * referenced daxdevs are already known (i.e. in the table). Any daxdev
+ * indices referenced in @meta but not in the table will be retrieved via
+ * famfs_fuse_get_daxdev() and added to the table
+ *
+ * Return: 0=success
+ *         -errno=failure
+ */
+static int
+famfs_update_daxdev_table(
+	struct fuse_mount *fm,
+	const struct famfs_file_meta *meta)
+{
+	struct famfs_dax_devlist *local_devlist;
+	struct fuse_conn *fc = fm->fc;
+	int indices_to_fetch[MAX_DAXDEVS];
+	int n_to_fetch = 0;
+	int err;
+
+	/* First time through we will need to allocate the dax_devlist */
+	if (!fc->dax_devlist) {
+		local_devlist = kcalloc(1, sizeof(*fc->dax_devlist), GFP_KERNEL);
+		if (!local_devlist)
+			return -ENOMEM;
+
+		local_devlist->nslots = MAX_DAXDEVS;
+
+		local_devlist->devlist = kcalloc(MAX_DAXDEVS,
+						 sizeof(struct famfs_daxdev),
+						 GFP_KERNEL);
+		if (!local_devlist->devlist) {
+			kfree(local_devlist);
+			return -ENOMEM;
+		}
+
+		/* We don't need famfs_devlist_sem here because we use cmpxchg */
+		if (cmpxchg(&fc->dax_devlist, NULL, local_devlist) != NULL) {
+			kfree(local_devlist->devlist);
+			kfree(local_devlist); /* another thread beat us to it */
+		}
+	}
+
+	/* Collect indices that need fetching while holding read lock */
+	scoped_guard(rwsem_read, &fc->famfs_devlist_sem) {
+		unsigned long i;
+
+		for_each_set_bit(i, (unsigned long *)&meta->dev_bitmap, MAX_DAXDEVS) {
+			if (!(fc->dax_devlist->devlist[i].valid))
+				indices_to_fetch[n_to_fetch++] = i;
+		}
+	}
+
+	/* Fetch needed daxdevs outside the read lock */
+	for (int j = 0; j < n_to_fetch; j++) {
+		err = famfs_fuse_get_daxdev(fm, indices_to_fetch[j]);
+		if (err)
+			pr_err("%s: failed to get daxdev=%d\n",
+			       __func__, indices_to_fetch[j]);
+	}
+
+	return 0;
+}
 
 /***************************************************************************/
 
@@ -184,7 +406,7 @@ famfs_fuse_meta_alloc(
 			/* ie_in = one interleaved extent in fmap_buf */
 			ie_in = fmap_buf + next_offset;
 
-			/* Move past one interleaved extent header in fmap_buf */
+			/* Move past 1 interleaved extent header in fmap_buf */
 			next_offset += sizeof(*ie_in);
 			if (next_offset > fmap_buf_size) {
 				pr_err("%s:%d: fmap_buf underflow offset/size %ld/%ld\n",
@@ -329,6 +551,9 @@ famfs_file_init_dax(
 	if (rc)
 		goto errout;
 
+	/* Make sure this fmap doesn't reference any unknown daxdevs */
+	famfs_update_daxdev_table(fm, meta);
+
 	/* Publish the famfs metadata on fi->famfs_meta */
 	inode_lock(inode);
 
diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
index 18ab22bcc5a1..eb9f70b5cb81 100644
--- a/fs/fuse/famfs_kfmap.h
+++ b/fs/fuse/famfs_kfmap.h
@@ -64,4 +64,30 @@ struct famfs_file_meta {
 	};
 };
 
+/*
+ * famfs_daxdev - tracking struct for a daxdev within a famfs file system
+ *
+ * This is the in-memory daxdev metadata that is populated by parsing
+ * the responses to GET_FMAP messages
+ */
+struct famfs_daxdev {
+	/* Include dev uuid? */
+	bool valid;
+	bool error;
+	dev_t devno;
+	struct dax_device *devp;
+	char *name;
+};
+
+#define MAX_DAXDEVS 24
+
+/*
+ * famfs_dax_devlist - list of famfs_daxdev's
+ */
+struct famfs_dax_devlist {
+	int nslots;
+	int ndevs;
+	struct famfs_daxdev *devlist;
+};
+
 #endif /* FAMFS_KFMAP_H */
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index dbfec5b9c6e1..83e24cee994b 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1006,6 +1006,11 @@ struct fuse_conn {
 		/* Request timeout (in jiffies). 0 = no timeout */
 		unsigned int req_timeout;
 	} timeout;
+
+#if IS_ENABLED(CONFIG_FUSE_FAMFS_DAX)
+	struct rw_semaphore famfs_devlist_sem;
+	struct famfs_dax_devlist *dax_devlist;
+#endif
 };
 
 /*
@@ -1647,6 +1652,8 @@ int famfs_file_init_dax(struct fuse_mount *fm,
 			size_t fmap_size);
 void __famfs_meta_free(void *map);
 
+void famfs_teardown(struct fuse_conn *fc);
+
 /* Set fi->famfs_meta = NULL regardless of prior value */
 static inline void famfs_meta_init(struct fuse_inode *fi)
 {
@@ -1668,6 +1675,11 @@ static inline void famfs_meta_free(struct fuse_inode *fi)
 	}
 }
 
+static inline void famfs_init_devlist_sem(struct fuse_conn *fc)
+{
+	init_rwsem(&fc->famfs_devlist_sem);
+}
+
 static inline int fuse_file_famfs(struct fuse_inode *fi)
 {
 	return (READ_ONCE(fi->famfs_meta) != NULL);
@@ -1677,6 +1689,9 @@ int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
 
 #else /* !CONFIG_FUSE_FAMFS_DAX */
 
+static inline void famfs_teardown(struct fuse_conn *fc)
+{
+}
 static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
 						  void *meta)
 {
@@ -1687,6 +1702,10 @@ static inline void famfs_meta_free(struct fuse_inode *fi)
 {
 }
 
+static inline void famfs_init_devlist_sem(struct fuse_conn *fc)
+{
+}
+
 static inline int fuse_file_famfs(struct fuse_inode *fi)
 {
 	return 0;
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index d0c6037ca46b..ecd83234c0e0 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1048,6 +1048,9 @@ void fuse_conn_put(struct fuse_conn *fc)
 		WARN_ON(atomic_read(&bucket->count) != 1);
 		kfree(bucket);
 	}
+	if (IS_ENABLED(CONFIG_FUSE_FAMFS_DAX))
+		famfs_teardown(fc);
+
 	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
 		fuse_backing_files_free(fc);
 	call_rcu(&fc->rcu, delayed_release);
@@ -1477,8 +1480,10 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
 				u64 in_flags = FIELD_PREP(GENMASK_ULL(63, 32), ia->in.flags2)
 						| ia->in.flags;
 
-				if (in_flags & FUSE_DAX_FMAP)
+				if (in_flags & FUSE_DAX_FMAP) {
+					famfs_init_devlist_sem(fc);
 					fc->famfs_iomap = 1;
+				}
 			}
 		} else {
 			ra_pages = fc->max_read / PAGE_SIZE;
diff --git a/fs/namei.c b/fs/namei.c
index 58f715f7657e..434081cfbaf7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4206,6 +4206,7 @@ bool may_open_dev(const struct path *path)
 	return !(path->mnt->mnt_flags & MNT_NODEV) &&
 		!(path->mnt->mnt_sb->s_iflags & SB_I_NODEV);
 }
+EXPORT_SYMBOL(may_open_dev);
 
 static int may_open(struct mnt_idmap *idmap, const struct path *path,
 		    int acc_mode, int flag)
diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
index cf678bebbfe0..1b82895108be 100644
--- a/include/uapi/linux/fuse.h
+++ b/include/uapi/linux/fuse.h
@@ -247,6 +247,9 @@
  *    - struct fuse_famfs_simple_ext
  *    - struct fuse_famfs_iext
  *    - struct fuse_famfs_fmap_header
+ *  - Add the following structs for the GET_DAXDEV message and reply
+ *    - struct fuse_get_daxdev_in
+ *    - struct fuse_get_daxdev_out
  *  - Add the following enumerated types
  *    - enum fuse_famfs_file_type
  *    - enum famfs_ext_type
@@ -678,6 +681,7 @@ enum fuse_opcode {
 
 	/* Famfs / devdax opcodes */
 	FUSE_GET_FMAP           = 54,
+	FUSE_GET_DAXDEV         = 55,
 
 	/* CUSE specific operations */
 	CUSE_INIT		= 4096,
@@ -1369,6 +1373,22 @@ struct fuse_famfs_fmap_header {
 	uint64_t reserved1;
 };
 
+struct fuse_get_daxdev_in {
+	uint32_t        daxdev_num;
+};
+
+#define DAXDEV_NAME_MAX 256
+
+/* fuse_daxdev_out has enough space for a uuid if we need it */
+struct fuse_daxdev_out {
+	uint16_t index;
+	uint16_t reserved;
+	uint32_t reserved2;
+	uint64_t reserved3;
+	uint64_t reserved4;
+	char name[DAXDEV_NAME_MAX];
+};
+
 static inline int32_t fmap_msg_min_size(void)
 {
 	/* Smallest fmap message is a header plus one simple extent */
-- 
2.53.0

[PATCH V9 06/10] famfs_fuse: Plumb dax iomap and fuse read/write/mmap

[John Groves]

From: John Groves <john@groves.net>

This commit fills in read/write/mmap handling for famfs files. The
dev_dax_iomap interface is used - just like xfs in fs-dax mode.

- Read/write are handled by famfs_fuse_[read|write]_iter() via
  dax_iomap_rw() to fsdev_dax.
- Mmap is handled by famfs_fuse_mmap()
- Faults are handled by famfs_filemap_fault(), using dax_iomap_fault()
  to fsdev_dax.
- File offset to dax offset resolution is handled via
  famfs_fuse_iomap_begin(), which uses famfs "fmaps" to resolve the
  the requested (file, offset) to an offset on a dax device (by way of
  famfs_fileofs_to_daxofs() and famfs_interleave_fileofs_to_daxofs())

Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/famfs.c  | 448 +++++++++++++++++++++++++++++++++++++++++++++++
 fs/fuse/file.c   |  18 +-
 fs/fuse/fuse_i.h |  19 ++
 3 files changed, 483 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index 0e9415aa6339..6f935032eb17 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -576,6 +576,454 @@ famfs_file_init_dax(
 	return rc;
 }
 
+/*********************************************************************
+ * iomap_operations
+ *
+ * This stuff uses the iomap (dax-related) helpers to resolve file offsets to
+ * offsets within a dax device.
+ */
+
+static int famfs_file_bad(struct inode *inode);
+
+static int
+famfs_interleave_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
+			 loff_t file_offset, off_t len, unsigned int flags)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct famfs_file_meta *meta = fi->famfs_meta;
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	loff_t local_offset = file_offset;
+
+	/* This function is only for extent_type INTERLEAVED_EXTENT */
+	if (meta->fm_extent_type != INTERLEAVED_EXTENT) {
+		pr_err("%s: bad extent type\n", __func__);
+		goto err_out;
+	}
+
+	if (famfs_file_bad(inode))
+		goto err_out;
+
+	iomap->offset = file_offset;
+
+	for (int i = 0; i < meta->fm_niext; i++) {
+		struct famfs_meta_interleaved_ext *fei = &meta->ie[i];
+		u64 chunk_size = fei->fie_chunk_size;
+		u64 nstrips = fei->fie_nstrips;
+		u64 ext_size =  min(fei->fie_nbytes, meta->file_size);
+
+		if (!IS_ALIGNED(chunk_size, PMD_SIZE)) {
+			pr_err("%s: chunk_size %lld not PMD-aligned\n",
+			       __func__, meta->ie[i].fie_chunk_size);
+			return -EINVAL;
+		}
+		if (ext_size == 0) {
+			pr_err("%s: ext_size=%lld file_size=%ld\n",
+			       __func__, fei->fie_nbytes, meta->file_size);
+			goto err_out;
+		}
+
+		/* Is the data is in this striped extent? */
+		if (local_offset < ext_size) {
+			u64 chunk_num       = local_offset / chunk_size;
+			u64 chunk_offset    = local_offset % chunk_size;
+			u64 chunk_remainder = chunk_size - chunk_offset;
+			u64 stripe_num      = chunk_num / nstrips;
+			u64 strip_num       = chunk_num % nstrips;
+			u64 strip_offset    = chunk_offset + (stripe_num * chunk_size);
+			u64 strip_dax_ofs = fei->ie_strips[strip_num].ext_offset;
+			u64 strip_devidx = fei->ie_strips[strip_num].dev_index;
+
+			if (strip_devidx >= fc->dax_devlist->nslots) {
+				pr_err("%s: strip_devidx %llu >= nslots %d\n",
+				       __func__, strip_devidx,
+				       fc->dax_devlist->nslots);
+				goto err_out;
+			}
+
+			if (!fc->dax_devlist->devlist[strip_devidx].valid) {
+				pr_err("%s: daxdev=%lld invalid\n", __func__,
+					strip_devidx);
+				goto err_out;
+			}
+
+			iomap->addr    = strip_dax_ofs + strip_offset;
+			iomap->offset  = file_offset;
+			iomap->length  = min_t(loff_t, len, chunk_remainder);
+
+			iomap->dax_dev = fc->dax_devlist->devlist[strip_devidx].devp;
+
+			iomap->type    = IOMAP_MAPPED;
+			iomap->flags   = flags;
+
+			return 0;
+		}
+		local_offset -= ext_size; /* offset is beyond this striped extent */
+	}
+
+ err_out:
+	pr_err("%s: err_out\n", __func__);
+
+	/* We fell out the end of the extent list.
+	 * Set iomap to zero length in this case, and return 0
+	 * This just means that the r/w is past EOF
+	 */
+	iomap->addr    = 0; /* there is no valid dax device offset */
+	iomap->offset  = file_offset; /* file offset */
+	iomap->length  = 0; /* this had better result in no access to dax mem */
+	iomap->dax_dev = NULL;
+	iomap->type    = IOMAP_MAPPED;
+	iomap->flags   = flags;
+
+	return -EIO;
+}
+
+/**
+ * famfs_fileofs_to_daxofs() - Resolve (file, offset, len) to (daxdev, offset, len)
+ *
+ * This function is called by famfs_fuse_iomap_begin() to resolve an offset in a
+ * file to an offset in a dax device. This is upcalled from dax from calls to
+ * both  * dax_iomap_fault() and dax_iomap_rw(). Dax finishes the job resolving
+ * a fault to a specific physical page (the fault case) or doing a memcpy
+ * variant (the rw case)
+ *
+ * Pages can be PTE (4k), PMD (2MiB) or (theoretically) PuD (1GiB)
+ * (these sizes are for X86; may vary on other cpu architectures
+ *
+ * @inode:  The file where the fault occurred
+ * @iomap:       To be filled in to indicate where to find the right memory,
+ *               relative  to a dax device.
+ * @file_offset: Within the file where the fault occurred (will be page boundary)
+ * @len:         The length of the faulted mapping (will be a page multiple)
+ *               (will be trimmed in *iomap if it's disjoint in the extent list)
+ * @flags:       flags passed to famfs_fuse_iomap_begin(), and sent back via
+ *               struct iomap
+ *
+ * Return values: 0. (info is returned in a modified @iomap struct)
+ */
+static int
+famfs_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
+			loff_t file_offset, off_t len, unsigned int flags)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct famfs_file_meta *meta = fi->famfs_meta;
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	loff_t local_offset = file_offset;
+
+	if (!fc->dax_devlist) {
+		pr_err("%s: null dax_devlist\n", __func__);
+		goto err_out;
+	}
+
+	if (famfs_file_bad(inode))
+		goto err_out;
+
+	if (meta->fm_extent_type == INTERLEAVED_EXTENT)
+		return famfs_interleave_fileofs_to_daxofs(inode, iomap,
+							  file_offset,
+							  len, flags);
+
+	iomap->offset = file_offset;
+
+	for (int i = 0; i < meta->fm_nextents; i++) {
+		/* TODO: check devindex too */
+		loff_t dax_ext_offset = meta->se[i].ext_offset;
+		loff_t dax_ext_len    = meta->se[i].ext_len;
+		u64 daxdev_idx = meta->se[i].dev_index;
+
+
+		/* TODO: test that superblock and log offsets only happen
+		 * with superblock and log files. Requires instrumentaiton
+		 * from user space...
+		 */
+
+		/* local_offset is the offset minus the size of extents skipped
+		 * so far; If local_offset < dax_ext_len, the data of interest
+		 * starts in this extent
+		 */
+		if (local_offset < dax_ext_len) {
+			loff_t ext_len_remainder = dax_ext_len - local_offset;
+			struct famfs_daxdev *dd;
+
+			if (daxdev_idx >= fc->dax_devlist->nslots) {
+				pr_err("%s: daxdev_idx %llu >= nslots %d\n",
+				       __func__, daxdev_idx,
+				       fc->dax_devlist->nslots);
+				goto err_out;
+			}
+
+			dd = &fc->dax_devlist->devlist[daxdev_idx];
+
+			if (!dd->valid || dd->error) {
+				pr_err("%s: daxdev=%lld %s\n", __func__,
+				       daxdev_idx,
+				       dd->valid ? "error" : "invalid");
+				goto err_out;
+			}
+
+			/*
+			 * OK, we found the file metadata extent where this
+			 * data begins
+			 * @local_offset      - The offset within the current
+			 *                      extent
+			 * @ext_len_remainder - Remaining length of ext after
+			 *                      skipping local_offset
+			 * Outputs:
+			 * iomap->addr:   the offset within the dax device where
+			 *                the  data starts
+			 * iomap->offset: the file offset
+			 * iomap->length: the valid length resolved here
+			 */
+			iomap->addr    = dax_ext_offset + local_offset;
+			iomap->offset  = file_offset;
+			iomap->length  = min_t(loff_t, len, ext_len_remainder);
+
+			iomap->dax_dev = fc->dax_devlist->devlist[daxdev_idx].devp;
+
+			iomap->type    = IOMAP_MAPPED;
+			iomap->flags   = flags;
+			return 0;
+		}
+		local_offset -= dax_ext_len; /* Get ready for the next extent */
+	}
+
+ err_out:
+	pr_err("%s: err_out\n", __func__);
+
+	/* We fell out the end of the extent list.
+	 * Set iomap to zero length in this case, and return 0
+	 * This just means that the r/w is past EOF
+	 */
+	iomap->addr    = 0; /* there is no valid dax device offset */
+	iomap->offset  = file_offset; /* file offset */
+	iomap->length  = 0; /* this had better result in no access to dax mem */
+	iomap->dax_dev = NULL;
+	iomap->type    = IOMAP_MAPPED;
+	iomap->flags   = flags;
+
+	return -EIO;
+}
+
+/**
+ * famfs_fuse_iomap_begin() - Handler for iomap_begin upcall from dax
+ *
+ * This function is pretty simple because files are
+ * * never partially allocated
+ * * never have holes (never sparse)
+ * * never "allocate on write"
+ *
+ * @inode:  inode for the file being accessed
+ * @offset: offset within the file
+ * @length: Length being accessed at offset
+ * @flags:  flags to be retured via struct iomap
+ * @iomap:  iomap struct to be filled in, resolving (offset, length) to
+ *          (daxdev, offset, len)
+ * @srcmap: source mapping if it is a COW operation (which it is not here)
+ */
+static int
+famfs_fuse_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
+		  unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct famfs_file_meta *meta = fi->famfs_meta;
+	size_t size;
+
+	size = i_size_read(inode);
+
+	WARN_ON(size != meta->file_size);
+
+	return famfs_fileofs_to_daxofs(inode, iomap, offset, length, flags);
+}
+
+/* Note: We never need a special set of write_iomap_ops because famfs never
+ * performs allocation on write.
+ */
+const struct iomap_ops famfs_iomap_ops = {
+	.iomap_begin		= famfs_fuse_iomap_begin,
+};
+
+/*********************************************************************
+ * vm_operations
+ */
+static vm_fault_t
+__famfs_fuse_filemap_fault(struct vm_fault *vmf, unsigned int pe_size,
+		      bool write_fault)
+{
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	vm_fault_t ret;
+	unsigned long pfn;
+
+	if (!IS_DAX(file_inode(vmf->vma->vm_file))) {
+		pr_err("%s: file not marked IS_DAX!!\n", __func__);
+		return VM_FAULT_SIGBUS;
+	}
+
+	if (write_fault) {
+		sb_start_pagefault(inode->i_sb);
+		file_update_time(vmf->vma->vm_file);
+	}
+
+	ret = dax_iomap_fault(vmf, pe_size, &pfn, NULL, &famfs_iomap_ops);
+	if (ret & VM_FAULT_NEEDDSYNC)
+		ret = dax_finish_sync_fault(vmf, pe_size, pfn);
+
+	if (write_fault)
+		sb_end_pagefault(inode->i_sb);
+
+	return ret;
+}
+
+static inline bool
+famfs_is_write_fault(struct vm_fault *vmf)
+{
+	return (vmf->flags & FAULT_FLAG_WRITE) &&
+	       (vmf->vma->vm_flags & VM_SHARED);
+}
+
+static vm_fault_t
+famfs_filemap_fault(struct vm_fault *vmf)
+{
+	return __famfs_fuse_filemap_fault(vmf, 0, famfs_is_write_fault(vmf));
+}
+
+static vm_fault_t
+famfs_filemap_huge_fault(struct vm_fault *vmf, unsigned int pe_size)
+{
+	return __famfs_fuse_filemap_fault(vmf, pe_size,
+					  famfs_is_write_fault(vmf));
+}
+
+static vm_fault_t
+famfs_filemap_mkwrite(struct vm_fault *vmf)
+{
+	return __famfs_fuse_filemap_fault(vmf, 0, true);
+}
+
+const struct vm_operations_struct famfs_file_vm_ops = {
+	.fault		= famfs_filemap_fault,
+	.huge_fault	= famfs_filemap_huge_fault,
+	.map_pages	= filemap_map_pages,
+	.page_mkwrite	= famfs_filemap_mkwrite,
+	.pfn_mkwrite	= famfs_filemap_mkwrite,
+};
+
+/*********************************************************************
+ * file_operations
+ */
+
+/**
+ * famfs_file_bad() - Check for files that aren't in a valid state
+ *
+ * @inode: inode
+ *
+ * Returns: 0=success
+ *          -errno=failure
+ */
+static int
+famfs_file_bad(struct inode *inode)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct famfs_file_meta *meta = fi->famfs_meta;
+	size_t i_size = i_size_read(inode);
+
+	if (!meta) {
+		pr_err("%s: un-initialized famfs file\n", __func__);
+		return -EIO;
+	}
+	if (meta->error) {
+		pr_debug("%s: previously detected metadata errors\n", __func__);
+		return -EIO;
+	}
+	if (i_size != meta->file_size) {
+		pr_warn("%s: i_size overwritten from %ld to %ld\n",
+		       __func__, meta->file_size, i_size);
+		meta->error = true;
+		return -ENXIO;
+	}
+	if (!IS_DAX(inode)) {
+		pr_debug("%s: inode %llx IS_DAX is false\n",
+			 __func__, (u64)inode);
+		return -ENXIO;
+	}
+	return 0;
+}
+
+static ssize_t
+famfs_fuse_rw_prep(struct kiocb *iocb, struct iov_iter *ubuf)
+{
+	struct inode *inode = iocb->ki_filp->f_mapping->host;
+	size_t i_size = i_size_read(inode);
+	size_t count = iov_iter_count(ubuf);
+	size_t max_count;
+	ssize_t rc;
+
+	rc = famfs_file_bad(inode);
+	if (rc)
+		return (ssize_t)rc;
+
+	/* Avoid unsigned underflow if position is past EOF */
+	if (iocb->ki_pos >= i_size)
+		max_count = 0;
+	else
+		max_count = i_size - iocb->ki_pos;
+
+	if (count > max_count)
+		iov_iter_truncate(ubuf, max_count);
+
+	if (!iov_iter_count(ubuf))
+		return 0;
+
+	return rc;
+}
+
+ssize_t
+famfs_fuse_read_iter(struct kiocb *iocb, struct iov_iter	*to)
+{
+	ssize_t rc;
+
+	rc = famfs_fuse_rw_prep(iocb, to);
+	if (rc)
+		return rc;
+
+	if (!iov_iter_count(to))
+		return 0;
+
+	rc = dax_iomap_rw(iocb, to, &famfs_iomap_ops);
+
+	file_accessed(iocb->ki_filp);
+	return rc;
+}
+
+ssize_t
+famfs_fuse_write_iter(struct kiocb *iocb, struct iov_iter *from)
+{
+	ssize_t rc;
+
+	rc = famfs_fuse_rw_prep(iocb, from);
+	if (rc)
+		return rc;
+
+	if (!iov_iter_count(from))
+		return 0;
+
+	return dax_iomap_rw(iocb, from, &famfs_iomap_ops);
+}
+
+int
+famfs_fuse_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct inode *inode = file_inode(file);
+	ssize_t rc;
+
+	rc = famfs_file_bad(inode);
+	if (rc)
+		return rc;
+
+	file_accessed(file);
+	vma->vm_ops = &famfs_file_vm_ops;
+	vm_flags_set(vma, VM_HUGEPAGE);
+	return 0;
+}
+
 #define FMAP_BUFSIZE PAGE_SIZE
 
 int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode)
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d7fd934f4412..e7e3d6aa2762 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1831,6 +1831,8 @@ static ssize_t fuse_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 
 	if (FUSE_IS_VIRTIO_DAX(fi))
 		return fuse_dax_read_iter(iocb, to);
+	if (fuse_file_famfs(fi))
+		return famfs_fuse_read_iter(iocb, to);
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
 	if (ff->open_flags & FOPEN_DIRECT_IO)
@@ -1853,6 +1855,8 @@ static ssize_t fuse_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 
 	if (FUSE_IS_VIRTIO_DAX(fi))
 		return fuse_dax_write_iter(iocb, from);
+	if (fuse_file_famfs(fi))
+		return famfs_fuse_write_iter(iocb, from);
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
 	if (ff->open_flags & FOPEN_DIRECT_IO)
@@ -1868,9 +1872,13 @@ static ssize_t fuse_splice_read(struct file *in, loff_t *ppos,
 				unsigned int flags)
 {
 	struct fuse_file *ff = in->private_data;
+	struct inode *inode = file_inode(in);
+	struct fuse_inode *fi = get_fuse_inode(inode);
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
-	if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
+	if (fuse_file_famfs(fi))
+		return -EIO; /* famfs does not use the page cache... */
+	else if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
 		return fuse_passthrough_splice_read(in, ppos, pipe, len, flags);
 	else
 		return filemap_splice_read(in, ppos, pipe, len, flags);
@@ -1880,9 +1888,13 @@ static ssize_t fuse_splice_write(struct pipe_inode_info *pipe, struct file *out,
 				 loff_t *ppos, size_t len, unsigned int flags)
 {
 	struct fuse_file *ff = out->private_data;
+	struct inode *inode = file_inode(out);
+	struct fuse_inode *fi = get_fuse_inode(inode);
 
 	/* FOPEN_DIRECT_IO overrides FOPEN_PASSTHROUGH */
-	if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
+	if (fuse_file_famfs(fi))
+		return -EIO; /* famfs does not use the page cache... */
+	else if (fuse_file_passthrough(ff) && !(ff->open_flags & FOPEN_DIRECT_IO))
 		return fuse_passthrough_splice_write(pipe, out, ppos, len, flags);
 	else
 		return iter_file_splice_write(pipe, out, ppos, len, flags);
@@ -2390,6 +2402,8 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
 	/* DAX mmap is superior to direct_io mmap */
 	if (FUSE_IS_VIRTIO_DAX(fi))
 		return fuse_dax_mmap(file, vma);
+	if (fuse_file_famfs(fi))
+		return famfs_fuse_mmap(file, vma);
 
 	/*
 	 * If inode is in passthrough io mode, because it has some file open
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 83e24cee994b..f5548466c2b2 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -1650,6 +1650,9 @@ extern void fuse_sysctl_unregister(void);
 int famfs_file_init_dax(struct fuse_mount *fm,
 			struct inode *inode, void *fmap_buf,
 			size_t fmap_size);
+ssize_t famfs_fuse_write_iter(struct kiocb *iocb, struct iov_iter *from);
+ssize_t famfs_fuse_read_iter(struct kiocb *iocb, struct iov_iter	*to);
+int famfs_fuse_mmap(struct file *file, struct vm_area_struct *vma);
 void __famfs_meta_free(void *map);
 
 void famfs_teardown(struct fuse_conn *fc);
@@ -1692,6 +1695,22 @@ int fuse_get_fmap(struct fuse_mount *fm, struct inode *inode);
 static inline void famfs_teardown(struct fuse_conn *fc)
 {
 }
+static inline ssize_t famfs_fuse_write_iter(struct kiocb *iocb,
+					    struct iov_iter *to)
+{
+	return -ENODEV;
+}
+static inline ssize_t famfs_fuse_read_iter(struct kiocb *iocb,
+					   struct iov_iter *to)
+{
+	return -ENODEV;
+}
+static inline int famfs_fuse_mmap(struct file *file,
+				  struct vm_area_struct *vma)
+{
+	return -ENODEV;
+}
+
 static inline struct fuse_backing *famfs_meta_set(struct fuse_inode *fi,
 						  void *meta)
 {
-- 
2.53.0

[PATCH V9 07/10] famfs_fuse: Add holder_operations for dax notify_failure()

[John Groves]

From: John Groves <john@groves.net>

Memory errors are at least somewhat more likely on disaggregated memory
than on-board memory. This commit registers to be notified by fsdev_dax
in the event that a memory failure is detected.

When a file access resolves to a daxdev with memory errors, it will fail
with an appropriate error.

If a daxdev failed fs_dax_get(), we set dd->dax_err. If a daxdev called
our notify_failure(), set dd->error. When any of the above happens, set
(file)->error and stop allowing access.

In general, the recovery from memory errors is to unmount the file
system and re-initialize the memory, but there may be usable degraded
modes of operation - particularly in the future when famfs supports
file systems backed by more than one daxdev. In those cases,
accessing data that is on a working daxdev can still work.

For now, return errors for any file that has encountered a memory or dax
error.

Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/famfs.c       | 110 +++++++++++++++++++++++++++++++++++++++---
 fs/fuse/famfs_kfmap.h |   3 +-
 2 files changed, 105 insertions(+), 8 deletions(-)

diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index 6f935032eb17..87012df537eb 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -21,6 +21,26 @@
 #include "famfs_kfmap.h"
 #include "fuse_i.h"
 
+static void famfs_set_daxdev_err(
+	struct fuse_conn *fc, struct dax_device *dax_devp);
+
+static int
+famfs_dax_notify_failure(struct dax_device *dax_devp, u64 offset,
+			u64 len, int mf_flags)
+{
+	struct fuse_conn *fc = dax_holder(dax_devp);
+
+	famfs_set_daxdev_err(fc, dax_devp);
+
+	return 0;
+}
+
+static const struct dax_holder_operations famfs_fuse_dax_holder_ops = {
+	.notify_failure		= famfs_dax_notify_failure,
+};
+
+/*****************************************************************************/
+
 /*
  * famfs_teardown()
  *
@@ -47,9 +67,12 @@ famfs_teardown(struct fuse_conn *fc)
 		if (!dd->valid)
 			continue;
 
-		/* Release reference from dax_dev_get() */
-		if (dd->devp)
+		/* Only call fs_put_dax if fs_dax_get succeeded */
+		if (dd->devp) {
+			if (!dd->dax_err)
+				fs_put_dax(dd->devp, fc);
 			put_dax(dd->devp);
+		}
 
 		kfree(dd->name);
 	}
@@ -169,6 +192,17 @@ famfs_fuse_get_daxdev(struct fuse_mount *fm, const u64 index)
 			return -ENODEV;
 		}
 
+		rc = fs_dax_get(daxdev->devp, fc, &famfs_fuse_dax_holder_ops);
+		if (rc) {
+			/* Mark as valid with dax_err to prevent retry loop.
+			 * famfs_dax_err() will return -EIO on access attempts.
+			 * Teardown handles this case: skips fs_put_dax, calls put_dax.
+			 */
+			daxdev->dax_err = 1;
+			pr_err("%s: fs_dax_get(%lld) failed\n",
+			       __func__, (u64)daxdev->devno);
+		}
+
 		wmb(); /* All other fields must be visible before valid */
 		daxdev->valid = 1;
 	}
@@ -244,6 +278,36 @@ famfs_update_daxdev_table(
 	return 0;
 }
 
+static void
+famfs_set_daxdev_err(
+	struct fuse_conn *fc,
+	struct dax_device *dax_devp)
+{
+	int i;
+
+	/* Gotta search the list by dax_devp;
+	 * read lock because we're not adding or removing daxdev entries
+	 */
+	scoped_guard(rwsem_write, &fc->famfs_devlist_sem) {
+		for (i = 0; i < fc->dax_devlist->nslots; i++) {
+			if (fc->dax_devlist->devlist[i].valid) {
+				struct famfs_daxdev *dd;
+
+				dd = &fc->dax_devlist->devlist[i];
+				if (dd->devp != dax_devp)
+					continue;
+
+				dd->error = true;
+
+				pr_err("%s: memory error on daxdev %s (%d)\n",
+				       __func__, dd->name, i);
+				return;
+			}
+		}
+	}
+	pr_err("%s: memory err on unrecognized daxdev\n", __func__);
+}
+
 /***************************************************************************/
 
 void __famfs_meta_free(void *famfs_meta)
@@ -585,6 +649,26 @@ famfs_file_init_dax(
 
 static int famfs_file_bad(struct inode *inode);
 
+static int famfs_dax_err(struct famfs_daxdev *dd)
+{
+	if (!dd->valid) {
+		pr_err("%s: daxdev=%s invalid\n",
+		       __func__, dd->name);
+		return -EIO;
+	}
+	if (dd->dax_err) {
+		pr_err("%s: daxdev=%s dax_err\n",
+		       __func__, dd->name);
+		return -EIO;
+	}
+	if (dd->error) {
+		pr_err("%s: daxdev=%s memory error\n",
+		       __func__, dd->name);
+		return -EHWPOISON;
+	}
+	return 0;
+}
+
 static int
 famfs_interleave_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
 			 loff_t file_offset, off_t len, unsigned int flags)
@@ -624,6 +708,7 @@ famfs_interleave_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
 
 		/* Is the data is in this striped extent? */
 		if (local_offset < ext_size) {
+			struct famfs_daxdev *dd;
 			u64 chunk_num       = local_offset / chunk_size;
 			u64 chunk_offset    = local_offset % chunk_size;
 			u64 chunk_remainder = chunk_size - chunk_offset;
@@ -632,6 +717,7 @@ famfs_interleave_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
 			u64 strip_offset    = chunk_offset + (stripe_num * chunk_size);
 			u64 strip_dax_ofs = fei->ie_strips[strip_num].ext_offset;
 			u64 strip_devidx = fei->ie_strips[strip_num].dev_index;
+			int rc;
 
 			if (strip_devidx >= fc->dax_devlist->nslots) {
 				pr_err("%s: strip_devidx %llu >= nslots %d\n",
@@ -646,6 +732,15 @@ famfs_interleave_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
 				goto err_out;
 			}
 
+			dd = &fc->dax_devlist->devlist[strip_devidx];
+
+			rc = famfs_dax_err(dd);
+			if (rc) {
+				/* Shut down access to this file */
+				meta->error = true;
+				return rc;
+			}
+
 			iomap->addr    = strip_dax_ofs + strip_offset;
 			iomap->offset  = file_offset;
 			iomap->length  = min_t(loff_t, len, chunk_remainder);
@@ -743,6 +838,7 @@ famfs_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
 		if (local_offset < dax_ext_len) {
 			loff_t ext_len_remainder = dax_ext_len - local_offset;
 			struct famfs_daxdev *dd;
+			int rc;
 
 			if (daxdev_idx >= fc->dax_devlist->nslots) {
 				pr_err("%s: daxdev_idx %llu >= nslots %d\n",
@@ -753,11 +849,11 @@ famfs_fileofs_to_daxofs(struct inode *inode, struct iomap *iomap,
 
 			dd = &fc->dax_devlist->devlist[daxdev_idx];
 
-			if (!dd->valid || dd->error) {
-				pr_err("%s: daxdev=%lld %s\n", __func__,
-				       daxdev_idx,
-				       dd->valid ? "error" : "invalid");
-				goto err_out;
+			rc = famfs_dax_err(dd);
+			if (rc) {
+				/* Shut down access to this file */
+				meta->error = true;
+				return rc;
 			}
 
 			/*
diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
index eb9f70b5cb81..0fff841f5a9e 100644
--- a/fs/fuse/famfs_kfmap.h
+++ b/fs/fuse/famfs_kfmap.h
@@ -73,7 +73,8 @@ struct famfs_file_meta {
 struct famfs_daxdev {
 	/* Include dev uuid? */
 	bool valid;
-	bool error;
+	bool error; /* Dax has reported a memory error (probably poison) */
+	bool dax_err; /* fs_dax_get() failed */
 	dev_t devno;
 	struct dax_device *devp;
 	char *name;
-- 
2.53.0

[PATCH V9 08/10] famfs_fuse: Add DAX address_space_operations with noop_dirty_folio

[John Groves]

From: John Groves <John@Groves.net>

Famfs is memory-backed; there is no place to write back to, and no
reason to mark pages dirty at all.

Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/famfs.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/fuse/famfs.c b/fs/fuse/famfs.c
index 87012df537eb..121ed74e9727 100644
--- a/fs/fuse/famfs.c
+++ b/fs/fuse/famfs.c
@@ -14,6 +14,7 @@
 #include <linux/mm.h>
 #include <linux/dax.h>
 #include <linux/iomap.h>
+#include <linux/pagemap.h>
 #include <linux/path.h>
 #include <linux/namei.h>
 #include <linux/string.h>
@@ -39,6 +40,15 @@ static const struct dax_holder_operations famfs_fuse_dax_holder_ops = {
 	.notify_failure		= famfs_dax_notify_failure,
 };
 
+/*
+ * DAX address_space_operations for famfs.
+ * famfs doesn't need dirty tracking - writes go directly to
+ * memory with no writeback required.
+ */
+static const struct address_space_operations famfs_dax_aops = {
+	.dirty_folio	= noop_dirty_folio,
+};
+
 /*****************************************************************************/
 
 /*
@@ -624,6 +634,7 @@ famfs_file_init_dax(
 	if (famfs_meta_set(fi, meta) == NULL) {
 		i_size_write(inode, meta->file_size);
 		inode->i_flags |= S_DAX;
+		inode->i_data.a_ops = &famfs_dax_aops;
 	} else {
 		pr_debug("%s: file already had metadata\n", __func__);
 		__famfs_meta_free(meta);
-- 
2.53.0

[PATCH V9 09/10] famfs_fuse: Add famfs fmap metadata documentation

[John Groves]

From: John Groves <John@Groves.net>

This describes the fmap metadata - both simple and interleaved

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: John Groves <john@groves.net>
---
 fs/fuse/famfs_kfmap.h | 73 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/fs/fuse/famfs_kfmap.h b/fs/fuse/famfs_kfmap.h
index 0fff841f5a9e..970ad802b492 100644
--- a/fs/fuse/famfs_kfmap.h
+++ b/fs/fuse/famfs_kfmap.h
@@ -7,6 +7,79 @@
 #ifndef FAMFS_KFMAP_H
 #define FAMFS_KFMAP_H
 
+/* KABI version 43 (aka v2) fmap structures
+ *
+ * The location of the memory backing for a famfs file is described by
+ * the response to the GET_FMAP fuse message (defined in
+ * include/uapi/linux/fuse.h
+ *
+ * There are currently two extent formats: Simple and Interleaved.
+ *
+ * Simple extents are just (devindex, offset, length) tuples, where devindex
+ * references a devdax device that must be retrievable via the GET_DAXDEV
+ * message/response.
+ *
+ * The extent list size must be >= file_size.
+ *
+ * Interleaved extents merit some additional explanation. Interleaved
+ * extents stripe data across a collection of strips. Each strip is a
+ * contiguous allocation from a single devdax device - and is described by
+ * a simple_extent structure.
+ *
+ * Interleaved_extent example:
+ *   ie_nstrips = 4
+ *   ie_chunk_size = 2MiB
+ *   ie_nbytes = 24MiB
+ *
+ * ┌────────────┐────────────┐────────────┐────────────┐
+ * │Chunk = 0   │Chunk = 1   │Chunk = 2   │Chunk = 3   │
+ * │Strip = 0   │Strip = 1   │Strip = 2   │Strip = 3   │
+ * │Stripe = 0  │Stripe = 0  │Stripe = 0  │Stripe = 0  │
+ * │            │            │            │            │
+ * └────────────┘────────────┘────────────┘────────────┘
+ * │Chunk = 4   │Chunk = 5   │Chunk = 6   │Chunk = 7   │
+ * │Strip = 0   │Strip = 1   │Strip = 2   │Strip = 3   │
+ * │Stripe = 1  │Stripe = 1  │Stripe = 1  │Stripe = 1  │
+ * │            │            │            │            │
+ * └────────────┘────────────┘────────────┘────────────┘
+ * │Chunk = 8   │Chunk = 9   │Chunk = 10  │Chunk = 11  │
+ * │Strip = 0   │Strip = 1   │Strip = 2   │Strip = 3   │
+ * │Stripe = 2  │Stripe = 2  │Stripe = 2  │Stripe = 2  │
+ * │            │            │            │            │
+ * └────────────┘────────────┘────────────┘────────────┘
+ *
+ * * Data is laid out across chunks in chunk # order
+ * * Columns are strips
+ * * Strips are contiguous devdax extents, normally each coming from a
+ *   different memory device
+ * * Rows are stripes
+ * * The number of chunks is (int)((file_size + chunk_size - 1) / chunk_size)
+ *   (and obviously the last chunk could be partial)
+ * * The stripe_size = (nstrips * chunk_size)
+ * * chunk_num(offset) = offset / chunk_size    //integer division
+ * * strip_num(offset) = chunk_num(offset) % nchunks
+ * * stripe_num(offset) = offset / stripe_size  //integer division
+ * * ...You get the idea - see the code for more details...
+ *
+ * Some concrete examples from the layout above:
+ * * Offset 0 in the file is offset 0 in chunk 0, which is offset 0 in
+ *   strip 0
+ * * Offset 4MiB in the file is offset 0 in chunk 2, which is offset 0 in
+ *   strip 2
+ * * Offset 15MiB in the file is offset 1MiB in chunk 7, which is offset
+ *   3MiB in strip 3
+ *
+ * Notes about this metadata format:
+ *
+ * * For various reasons, chunk_size must be a multiple of the applicable
+ *   PAGE_SIZE
+ * * Since chunk_size and nstrips are constant within an interleaved_extent,
+ *   resolving a file offset to a strip offset within a single
+ *   interleaved_ext is order 1.
+ * * If nstrips==1, a list of interleaved_ext structures degenerates to a
+ *   regular extent list (albeit with some wasted struct space).
+ */
+
 /*
  * The structures below are the in-memory metadata format for famfs files.
  * Metadata retrieved via the GET_FMAP response is converted to this format
-- 
2.53.0

[PATCH V9 10/10] famfs_fuse: Add documentation

[John Groves]

From: John Groves <john@groves.net>

Add Documentation/filesystems/famfs.rst and update MAINTAINERS

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: John Groves <john@groves.net>
---
 Documentation/filesystems/famfs.rst | 142 ++++++++++++++++++++++++++++
 Documentation/filesystems/index.rst |   1 +
 MAINTAINERS                         |   1 +
 3 files changed, 144 insertions(+)
 create mode 100644 Documentation/filesystems/famfs.rst

diff --git a/Documentation/filesystems/famfs.rst b/Documentation/filesystems/famfs.rst
new file mode 100644
index 000000000000..d90ce96d6fda
--- /dev/null
+++ b/Documentation/filesystems/famfs.rst
@@ -0,0 +1,142 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _famfs_index:
+
+==================================================================
+famfs: The fabric-attached memory file system
+==================================================================
+
+- Copyright (C) 2024-2026 Micron Technology, Inc.
+
+Introduction
+============
+Compute Express Link (CXL) provides a mechanism for disaggregated or
+fabric-attached memory (FAM). This creates opportunities for data sharing;
+clustered apps that would otherwise have to shard or replicate data can
+share one copy in disaggregated memory.
+
+Famfs, which is not CXL-specific in any way, provides a mechanism for
+multiple hosts to concurrently access data in shared memory, by giving it
+a file system interface. With famfs, any app that understands files can
+access data sets in shared memory. Although famfs supports read and write,
+the real point is to support mmap, which provides direct (dax) access to
+the memory - either writable or read-only.
+
+Shared memory can pose complex coherency and synchronization issues, but
+there are also simple cases. Two simple and eminently useful patterns that
+occur frequently in data analytics and AI are:
+
+* Serial Sharing - Only one host or process at a time has access to a file
+* Read-only Sharing - Multiple hosts or processes share read-only access
+  to a file
+
+The famfs fuse file system is part of the famfs framework; user space
+components [1] handle metadata allocation and distribution, and provide a
+low-level fuse server to expose files that map directly to [presumably
+shared] memory.
+
+The famfs framework manages coherency of its own metadata and structures,
+but does not attempt to manage coherency for applications.
+
+Famfs also provides data isolation between files. That is, even though
+the host has access to an entire memory "device" (as a devdax device), apps
+cannot write to memory for which the file is read-only, and mapping one
+file provides isolation from the memory of all other files. This is pretty
+basic, but some experimental shared memory usage patterns provide no such
+isolation.
+
+Principles of Operation
+=======================
+
+Famfs is a file system with one or more devdax devices as a first-class
+backing device(s). Metadata maintenance and query operations happen
+entirely in user space.
+
+The famfs low-level fuse server daemon provides file maps (fmaps) and
+devdax device info to the fuse/famfs kernel component so that
+read/write/mapping faults can be handled without up-calls for all active
+files.
+
+The famfs user space is responsible for maintaining and distributing
+consistent metadata. This is currently handled via an append-only
+metadata log within the memory, but this is orthogonal to the fuse/famfs
+kernel code.
+
+Once instantiated, "the same file" on each host points to the same shared
+memory, but in-memory metadata (inodes, etc.) is ephemeral on each host
+that has a famfs instance mounted. Use cases are free to allow or not
+allow mutations to data on a file-by-file basis.
+
+When an app accesses a data object in a famfs file, there is no page cache
+involvement. The CPU cache is loaded directly from the shared memory. In
+some use cases, this is an enormous reduction in read amplification
+compared to loading an entire page into the page cache.
+
+
+Famfs is Not a Conventional File System
+---------------------------------------
+
+Famfs files can be accessed by conventional means, but there are
+limitations. The kernel component of fuse/famfs is not involved in the
+allocation of backing memory for files at all; the famfs user space
+creates files and responds as a low-level fuse server with fmaps and
+devdax device info upon request.
+
+Famfs differs in some important ways from conventional file systems:
+
+* Files must be pre-allocated by the famfs framework; allocation is never
+  performed on (or after) write.
+* Any operation that changes a file's size is considered to put the file
+  in an invalid state, disabling access to the data. It may be possible to
+  revisit this in the future. (Typically the famfs user space can restore
+  files to a valid state by replaying the famfs metadata log.)
+
+Famfs exists to apply the existing file system abstractions to shared
+memory so applications and workflows can more easily adapt to an
+environment with disaggregated shared memory.
+
+Memory Error Handling
+=====================
+
+Possible memory errors include timeouts, poison, and unexpected
+reconfiguration of an underlying dax device. In all of these cases, famfs
+receives a call from the devdax layer via its iomap_ops->notify_failure()
+function. If any memory errors have been detected, access to the affected
+daxdev is disabled to avoid further errors or corruption.
+
+In all known cases, famfs can be unmounted cleanly. In most cases errors
+can be cleared by re-initializing the memory - at which point a new famfs
+file system can be created.
+
+Key Requirements
+================
+
+The primary requirements for famfs are:
+
+1. Must support a file system abstraction backed by sharable devdax memory
+2. Files must efficiently handle VMA faults
+3. Must support metadata distribution in a sharable way
+4. Must handle clients with a stale copy of metadata
+
+The famfs kernel component takes care of 1-2 above by caching each file's
+mapping metadata in the kernel.
+
+Requirements 3 and 4 are handled by the user space components, and are
+largely orthogonal to the functionality of the famfs kernel module.
+
+Requirements 3 and 4 cannot be met by conventional fs-dax file systems
+(e.g. xfs) because they use write-back metadata; it is not valid to mount
+such a file system on two hosts from the same in-memory image.
+
+
+Famfs Usage
+===========
+
+Famfs usage is documented at [1].
+
+
+References
+==========
+
+- [1] Famfs user space repository and documentation
+      https://github.com/cxl-micron-reskit/famfs
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index f4873197587d..e6fb467c1680 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -89,6 +89,7 @@ Documentation for filesystem implementations.
    ext3
    ext4/index
    f2fs
+   famfs
    gfs2/index
    hfs
    hfsplus
diff --git a/MAINTAINERS b/MAINTAINERS
index c590988881f6..b7c43262c2d3 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10518,6 +10518,7 @@ M:	John Groves <John@Groves.net>
 L:	linux-cxl@vger.kernel.org
 L:	linux-fsdevel@vger.kernel.org
 S:	Supported
+F:	Documentation/filesystems/famfs.rst
 F:	fs/fuse/famfs.c
 F:	fs/fuse/famfs_kfmap.h
 
-- 
2.53.0