Re: [RFC V2 18/18] famfs_fuse: Add documentation

[Amir Goldstein]

On Thu, Jul 3, 2025 at 8:51 PM John Groves <John@xxxxxxxxxx> wrote:
>
> Add Documentation/filesystems/famfs.rst and update MAINTAINERS
>
> Signed-off-by: John Groves <john@xxxxxxxxxx>
> ---
>  Documentation/filesystems/famfs.rst | 142 ++++++++++++++++++++++++++++
>  Documentation/filesystems/index.rst |   1 +
>  MAINTAINERS                         |   1 +
>  3 files changed, 144 insertions(+)
>  create mode 100644 Documentation/filesystems/famfs.rst


Considering "Documentation: fuse: Consolidate FUSE docs into its own
subdirectory"
https://lore.kernel.org/linux-fsdevel/20250612032239.17561-1-bagasdotme@xxxxxxxxx/

I wonder if famfs and virtiofs should be moved into fuse subdir?
To me it makes more sense, but it's not a clear cut.

>
> diff --git a/Documentation/filesystems/famfs.rst b/Documentation/filesystems/famfs.rst
> new file mode 100644
> index 000000000000..0d3c9ba9b7a8
> --- /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) 2024-2025 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 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 2636f2a41bd3..5aad315206ee 100644
> --- a/Documentation/filesystems/index.rst
> +++ b/Documentation/filesystems/index.rst
> @@ -90,6 +90,7 @@ Documentation for filesystem implementations.
>     ext3
>     ext4/index
>     f2fs
> +   famfs
>     gfs2
>     gfs2-uevents
>     gfs2-glocks
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 02688f27a4d0..faa7de4a43de 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -8814,6 +8814,7 @@ M:        John Groves <John@xxxxxxxxxx>
>  L:     linux-cxl@xxxxxxxxxxxxxxx
>  L:     linux-fsdevel@xxxxxxxxxxxxxxx
>  S:     Supported
> +F:     Documentation/filesystems/famfs.rst
>  F:     fs/fuse/famfs.c
>  F:     fs/fuse/famfs_kfmap.h
>
> --
> 2.49.0
>