Add Documentation/filesystems/famfs.rst and update MAINTAINERS Signed-off-by: John Groves <j...@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 <j...@groves.net> L: linux-...@vger.kernel.org L: linux-fsde...@vger.kernel.org S: Supported +F: Documentation/filesystems/famfs.rst F: fs/fuse/famfs.c F: fs/fuse/famfs_kfmap.h -- 2.49.0
