From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7E0B831F9A5; Thu, 6 Aug 2026 05:38:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785994734; cv=none; b=E4hINVvtsE2n4Lh7xkAtB4NL7xY5zaB/T1gZT3gcz5QzBjglpMuZpsBp6yjAtJkuimdUN11z1msxOEtdS91HMSmYvNaV9AVAeY3hS2W5JJe2y9n3XOWu9OjmrhT03wCF59tOMFgl8OnqiMtRkgcDXdIYUg5G6wkJHkLj9fFmtMM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785994734; c=relaxed/simple; bh=L81Gr6P5a728WMvMFjKLgHLP45lr7qAP9Y0Aifk120U=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Ls12DGzNLtUjEUo2H6waOvTFHo6LtokwPIuqV/M0uX82dlTSryeDs1Zr1MvpkWOsO/BY1FpR3zjya0jrSgfUG6pWBffBYp0EFOpQdQ/yH8AHwL8Jr6XpJbyy+e1OGED4ClAMKk+X4oLf4bp10oTHAF86/EwCPkT25ydU23q62xo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=edLsjiiz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="edLsjiiz" Received: by smtp.kernel.org (Postfix) with UTF8SMTPSA id 1A2401F000E9; Thu, 6 Aug 2026 05:38:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785994733; bh=S1IfjQLdDCOc5kDbzKWnjvR3WajSSqGNKDzYKfGSTgE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=edLsjiiz6/oX1ELsrLrTXtUabnLdZ3cD8XXabOMZQoPzMhzKjLOsPVFclMTOxZdk5 c1FlBtpYmLQqo7qCD0ud7WG1tD/LOTywCs9sbAZd4DvoT7z+LvWpbQUYGYgEQbnoDq d3sIcHJ+KMZyRMrFfpoO5LqdQb6DqB3bRfnFktJrEsg27zycXkBygGNYtu0uSdu+0e 36ykJAgPOlpAfi0mUAh+gP9wx6ZAl01Aiy24VQ0/ke96/O32hP6idZE+iQE1rAD4O/ K7bKz+Sp+oVd4XUOM8OGUNPOUWeOYkyKADNEQqSOR1cLw0WxB2qbJvHwBRlW7M+rov Rp2rKkfmQI05g== Date: Wed, 5 Aug 2026 22:38:52 -0700 From: "Darrick J. Wong" To: John Groves Cc: John Groves , Miklos Szeredi , Dan Williams , Bernd Schubert , Alison Schofield , John Groves , Jonathan Corbet , Jake Edge , Shuah Khan , Vishal Verma , Dave Jiang , Matthew Wilcox , Jan Kara , Alexander Viro , David Hildenbrand , Christian Brauner , Randy Dunlap , Jeff Layton , Amir Goldstein , Jonathan Cameron , Stefan Hajnoczi , Joanne Koong , Josef Bacik , Bagas Sanjaya , Chen Linxuan , James Morse , Fuad Tabba , Sean Christopherson , Shivank Garg , Ackerley Tng , Gregory Price , Andrew Morton , Namjae Jeon , Lorenzo Stoakes , Greg Kroah-Hartman , Ira Weiny , Pasha Tatashin , Haren Myneni , Pratyush Yadav , Giovanni Cabiddu , Jiri Slaby , Ethan Nelson-Moore , Gabriel Whigham , Aravind Ramesh , Ajay Joshi , "venkataravis@micron.com" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "nvdimm@lists.linux.dev" , "linux-cxl@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "fuse-devel@lists.linux.dev" , Jonathan Cameron Subject: Re: [PATCH V12 12/12] famfs: Add documentation Message-ID: <20260806053852.GM3560084@frogsfrogsfrogs> References: <0100019fc572ca94-ec363dd7-3a77-484b-b4b7-f2503a0931a6-000000@email.amazonses.com> <20260803023009.75966-1-john@jagalactic.com> <0100019fc5752a8f-df47f869-a29b-4b6d-936a-67dd521c0f5e-000000@email.amazonses.com> Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0100019fc5752a8f-df47f869-a29b-4b6d-936a-67dd521c0f5e-000000@email.amazonses.com> On Mon, Aug 03, 2026 at 02:30:18AM +0000, John Groves wrote: > From: John Groves > > Add Documentation/filesystems/famfs.rst and update MAINTAINERS > > Reviewed-by: Randy Dunlap > Tested-by: Randy Dunlap > Reviewed-by: Jonathan Cameron > Signed-off-by: John Groves > --- > 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..048791bcbcfd > --- /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) 2024-2026 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 file system is part of the famfs framework; user space > +components [1] handle metadata allocation and distribution, and populate > +the in-kernel files (via ioctls) so that they map directly to shared > +memory. Ok, so maybe it is the famfs server that's supposed to go modify the directory tree after the metadata log updates. > +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 user space provides each file's map (fmap) and its backing > +devdax devices to the kernel through ioctls (FAMFSIOC_MAP_CREATE and > +FAMFSIOC_DAXDEV_OPEN), after which read/write/mapping faults are handled > +entirely in the kernel with no 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 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 in 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 famfs kernel module is not involved in the allocation of > +backing memory for files at all; the famfs user space creates files and > +supplies their fmaps and devdax device info to the kernel via ioctls. ...and just to be clear, regular programs *do not* modify the directory tree, create files, or delete them? > +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 > +dax_holder_operations->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. Should media failure notifications get passed to the new fserror framework (see xfs_verify_media.c)? --D > +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 1f71cf159547..97a9a1f1e96a 100644 > --- a/Documentation/filesystems/index.rst > +++ b/Documentation/filesystems/index.rst > @@ -91,6 +91,7 @@ Documentation for filesystem implementations. > ext3 > ext4/index > f2fs > + famfs > gfs2/index > hfs > hfsplus > diff --git a/MAINTAINERS b/MAINTAINERS > index ca7b90a8f0a1..8c282857b103 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -9910,6 +9910,7 @@ M: John Groves > L: linux-fsdevel@vger.kernel.org > L: linux-cxl@vger.kernel.org > S: Supported > +F: Documentation/filesystems/famfs.rst > F: fs/famfs/ > > FANOTIFY > -- > 2.53.0 > > >