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 >
