[RFC PATCH 18/19] famfs_fuse: Add documentation

[John Groves <John@Groves.net>]

Add Documentation/filesystems/famfs.rst and update MAINTAINERS

Signed-off-by: John Groves <john@groves.net>
---
 Documentation/filesystems/famfs.rst | 142 ++++++++++++++++++++++++++++
 Documentation/filesystems/index.rst |   1 +
 MAINTAINERS                         |   1 +
 3 files changed, 144 insertions(+)
 create mode 100644 Documentation/filesystems/famfs.rst

diff --git a/Documentation/filesystems/famfs.rst b/Documentation/filesystems/famfs.rst
new file mode 100644
index 000000000000..b6b3500b6905
--- /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 2a5a7e0e8b28..46744be9e6d1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8814,6 +8814,7 @@ M:	John Groves <John@Groves.net>
 L:	linux-cxl@vger.kernel.org
 L:	linux-fsdevel@vger.kernel.org
 S:	Supported
+F:	Documentation/filesystems/famfs.rst
 F:	fs/fuse/famfs.c
 F:	fs/fuse/famfs_kfmap.h
 
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2.49.0