On 25/04/21 07:10PM, Randy Dunlap wrote: > > > On 4/20/25 6:33 PM, John Groves wrote: > > 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 > > user > > > +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 > > allocation > > > + 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. > > > -- > ~Randy >
Both edits applied to the -next branch for the patch set. Thanks!
