linux-kernel - [PATCH BUNDLE v3] famfs: Fabric-Attached Memory File System

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Message-ID: <0100019bc82c7f86-fa3e7ed1-6abc-40b2-b73c-d3021ea16342-000000@email.amazonses.com>
Date: Fri, 16 Jan 2026 18:58:38 +0000
From: John Groves <john@...alactic.com>
To: John Groves <John@...ves.net>, 
	Miklos Szeredi <miklos@...redi.hu>, 
	Dan Williams <dan.j.williams@...el.com>, 
	Bernd Schubert <bschubert@....com>, 
	Alison Schofield <alison.schofield@...el.com>
Cc: John Groves <jgroves@...ron.com>, 
	John Groves <jgroves@...tmail.com>, 
	Jonathan Corbet <corbet@....net>, 
	Vishal Verma <vishal.l.verma@...el.com>, 
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	Jan Kara <jack@...e.cz>, 
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Subject: [PATCH BUNDLE v3] famfs: Fabric-Attached Memory File System

[no change since last version - gmail dropped some of those messages]

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

  1. Linux kernel (21 patches) - dax fsdev driver + fuse/famfs integration
  2. libfuse (4 patches) - famfs protocol support for fuse servers
  3. ndctl/daxctl (2 patches) - support for the new "famfs" devdax mode
     (this is unchanged, but marked V2 to keep the "bundle" together)

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 (V3, 21 patches):
  - dax: 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.
  - fuse: Famfs integration adding GET_FMAP and GET_DAXDEV messages for
    caching file-to-dax mappings in the kernel.

libfuse (V2, 4 patches):
  - Updates fuse_kernel.h to kernel 6.19 baseline
  - Adds famfs DAX fmap protocol definitions
  - Implements famfs DAX fmap support for fuse servers

ndctl/daxctl (2 patches):
  - Adds daxctl support for the new "famfs" mode of devdax
  - Adds test/daxctl-famfs.sh for testing mode transitions

Changes Since V2 (kernel)
-------------------------
- Dax: Completely new fsdev driver replaces the dev_dax_iomap modifications.
  Uses MEMORY_DEVICE_FS_DAX type with order-0 folios for fs-dax compatibility.
- Dax: The "poisoned page" problem is properly fixed via fsdev_clear_folio_state()
  which clears stale mapping/compound state when fsdev binds.
- Dax: Added dax_set_ops() and driver unbind protection while filesystem mounted.
- Fuse: Famfs mounts require CAP_SYS_RAWIO (exposing raw memory devices).
- Fuse: Added DAX address_space_operations with noop_dirty_folio.
- Rebased to latest kernels, compatible with recent dax refactoring.

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