[PATCH BUNDLE v8] famfs: Fabric-Attached Memory File System

[John Groves <john@groves.net>]

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

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

Note: in this version I split the kernel dax patches into a separate
series from the kernel fuse patches. This is because those go through
separate maintainers. The plan is for the dax patches to go into Ira's
dax tree, and for Miklos to pull in that branch and apply the fuse series
on top if it.

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