[PATCH V10 00/10] famfs: port into fuse

John Groves
2026-03-31T12:37:18Z

From: John Groves <john@groves.net>

NOTE: this series depends on the famfs dax series in Ira's for-7.1/dax-famfs
branch [0]

Changes v9 -> v10
- Rebased to Ira's for-7.1/dax-famfs branch [0], which contains the required
dax patches
- Add parentheses to FUSE_IS_VIRTIO_DAX() macro, in case something bad is
passed in as fuse_inode (thanks Jonathan's AI)

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 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...)

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://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm.git/
[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: 2ae624d5a555d47a735fb3f4d850402859a4db77
--
2.53.0