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

John Groves
2026-04-10T18:38:49Z

On 26/04/10 05:24PM, Bernd Schubert wrote:
>
>
> On 4/10/26 16:46, John Groves wrote:
> > On 26/04/06 10:43AM, Joanne Koong wrote:
> >> On Tue, Mar 31, 2026 at 5:37 AM John Groves <john@jagalactic.com> wrote:
> >>>
> >>> 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
> >>>
> >>
> >> Hi John,
> >>
> >> I’m curious to hear your thoughts on whether you think it makes sense
> >> for the famfs-specific logic in this series to be moved to a bpf
> >> program that goes through a generic fuse iomap dax layer.
> >>
> >> Based on [1], this gives feature-parity with the famfs logic in this
> >> series. In my opinion, having famfs go through a generic fuse iomap
> >> dax layer makes the fuse kernel code more extensible for future
> >> servers that will also want to use dax iomap, and keeps the fuse code
> >> cleaner by not having famfs-specific logic hardcoded in and having to
> >> introduce new fuse uapis for something famfs-specific. In my
> >> understanding of it, fuse is meant to be generic and it feels like
> >> adding server-specific logic goes against that design philosophy and
> >> sets a precedent for other servers wanting similar special-casing in
> >> the future. I'd like to explore whether the bpf and generic fuse iomap
> >> dax layer approach can preserve that philosophy while still giving
> >> famfs the flexibility it needs.
> >>
> >> I think moving the famfs logic to bpf benefits famfs as well:
> >> - Instead of needing to issue a FUSE_GET_FMAP request after a file is
> >> opened, the server can directly populate the metadata map from
> >> userspace with the mapping info when it processes the FUSE_OPEN
> >> request, which gets rid of the roundtrip cost
> >> - The server can dynamically update the metadata / bpf maps during
> >> runtime from userspace if any mapping info needs to change
> >> - Future code changes / updates for famfs are all server-side and can
> >> be deployed immediately instead of needing to go through the upstream
> >> kernel mailing list process
> >> - Famfs updates / new releases can ship independently of kernel releases
> >>
> >> I'd appreciate the chance to discuss tradeoffs or if you'd rather
> >> discuss this at the fuse BoF at lsf, that sounds great too.
> >>
> >> Thanks,
> >> Joanne
> >>
> >
> > Hi Joanne,
> >
> > I'm definitely up for discussing it, and talking before LSFMM would be
> > good because then I'd have some time to think about before we discuss
> > at LSFMM.
> >
> > I have not had a chance to really study this, in part since I've never even
> > written a "hello world" BPF program.
> >
> > I'll ping off-list about times to talk.
> >
> > However...
> >
> > I would object vehemently to this sort of re-write prior to going upstream,
> > as would users and vendors who need famfs now that the memory products it
> > enables have started to ship.
> >
> > This work started over 3 years ago, initial patches over 2 years ago,
> > community decision that it should go into fuse 2 years ago, first fuse
> > patches a year ago.
> >
> > This implementation is pretty much exactly in line with expectation-setting
> > starting two years ago. Famfs is a complicated orchestration between dax,
> > fuse, ndctl (for daxctl), libfuse and the extensive famfs user space. Famfs
> > has a fairly small kernel footprint, but its user space is much larger.
> > This could set it back a year if we re-write now.
> >
> > Two things are true at once: I think this is a serious idea worth
> > considering, and I think it's too late to make this sort of change before
> > going upstream. Products need this enablement, and quite a long process has
> > run in order to make it available in a timely fashion (which means soon
> > now). I hope we can avoid making the perfect the enemy of the good.
> >
> > I believe the risk of merging famfs soon is quite low, because famfs will
> > not affect anybody who doesn't use it. I hope we can run this discussion and
> > analysis in parallel with merging the current implementation of famfs soon.
>
>
> Hi John,
>
> one question, assuming most of these things can be done with eBPF, would
> you convert to eBPF after the merge?

Hi Bernd,

Stipulating that I've never even written 'hello world' with BPF, if it's
a nicer solution with no downsides we would migrate there. I don't know
enough yet to put a time frame on it, but I'll certainly understand more
by LSFMM. Will you be there?

I'm hoping for a call with Joanne and Darrick in the next few days to get
better educated on it.

>
> (I also need to find the time to review at least all of your libfuse
> changes, I feel guilty that still haven't done it.).
>
>
> Thanks,
> Bernd

The libfuse changes are pretty small now. Two new messages - GET_FMAP and
GET_DAXDEV - plus a few more bits and bobs. If a future BPF migration took
place, there is a chance that those message numbers could be retired and
reused in the future.

Watch this space...

Thanks,
John