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 10DFD442FB9; Thu, 20 Aug 2026 12:37:18 +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=1787229440; cv=none; b=GHDzJaZBrd7yiRc0RAXaHf5Hi9/5rx6OVoog+AhwtKbrtEdI1nwZWlrZ6DVeO93XCWnx2RcJH+mFKxqvg4bJSQW+BzObra3xEnfCIWEQBYc8z4zWubkBveAXKkuRDkvks4eo88jmMIp+OFZUf56vYU7CuDYeXPCZKOJeSp5nupY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787229440; c=relaxed/simple; bh=Xo5H0uKvYwj+XAKUm2yiMtUKonkVDS/xrj/m6w9z0SI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=sw+ig+79zZrv3UHBOt7V8zF6yvuaSWCB+zR34keP2kcZbVrx085TN2z/vqreRq+oImLagUQZ5ooSoZHRCqcpGvzJR7ezLN2fkRYRDSYDhDZIp5v6ER3SvCfqMZJmz0N1ti/o80yDsxZuCWF59ULjEkYhqIQduHm39MdtYqFXvd0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GUbHW4aL; 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="GUbHW4aL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CAE1E1F000E9; Thu, 20 Aug 2026 12:37:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787229438; bh=OQpLOvB54LuGw8DzYe60oPZGDZAiKE8dvH2B4xQo3I4=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=GUbHW4aLYVRspV94M1rzQdL4joanhEC4j6cAyaqhy870zFoAEh4BFZg9S1XTqvfRu 8liJ9S9xzTOo5vwTTbIw90lgo5WfXD8Zrvyme19y1dz8VPKnn9g8+pGzopSQGdD9zN 34ZQI35a9gEE6ZkGxGJnmGQUCfDYGmrFJ9Y8/cTPE5wD3xn1tz30hDr9qvJ/ySNCiw 6boTc+/NT3GWrA9R1UhE9aAAj05ZBzVB5AXNQ6viYu8+c2U9oraZRNk+QoVqXwMsKq Nug+g1AmtNKOCmqzlsJ+g09AAGTZ7bDOi0ODQCq4yGjYzG3UMysKtegJuO0x7i4Qug CdDy4CAwUCO9g== Message-ID: <1f218de3db5bb7a89e56bb100fd4a4b87e5dc536.camel@kernel.org> Subject: Re: [PATCH v13 00/12] famfs: the Fabric-Attached Memory File System (standalone) From: Jeff Layton To: John Groves , John Groves , Miklos Szeredi , Dan Williams , Bernd Schubert , Alison Schofield Cc: John Groves , Jonathan Corbet , Jake Edge , Shuah Khan , Vishal Verma , Dave Jiang , Matthew Wilcox , Jan Kara , Alexander Viro , David Hildenbrand , Christian Brauner , "Darrick J . Wong" , Randy Dunlap , 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" Date: Thu, 20 Aug 2026 08:37:13 -0400 In-Reply-To: <0100019fed5850ec-2bdfb17a-3086-44ea-8fdd-777d3ce12a33-000000@email.amazonses.com> References: <20260810202325.96202-1-john@jagalactic.com> <0100019fed5850ec-2bdfb17a-3086-44ea-8fdd-777d3ce12a33-000000@email.amazonses.com> Autocrypt: addr=jlayton@kernel.org; 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text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2026-08-10 at 20:23 +0000, John Groves wrote: > From: John Groves >=20 > This is v13 of famfs, as a standalone file system. v12 is at [7]. >=20 > Most of this cover letter repeats v12. If you have read it, skip to "Chan= ges > since v12" below and to the per-patch changelogs under the "---" line of = each > patch. The history goes back to 2023, and is briefly covered in the v12 > cover. >=20 > The most important thing to know about famfs is that it CANNOT be used as= a > general purpose file system. It is for enabling file-based byte-level acc= ess > (including direct mmap) to very large (e.g. 100TB) shared/disaggregated > memory appliances - which have become available during this long process, > and which are in need of Linux support. >=20 > So famfs cannot be used by anybody who doesn't know why they need it. Mak= ing > famfs standalone means it can't affect users who don't use it. Micron, as > well as other memory companies, need it, and do not intend to abandon it = - > but imagine if we did: fs/famfs/ should be removed if the memory companie= s > can't be bothered to maintain it. >=20 > This code base has been in active CI and used globally by early adopters = and > testers of disaggregated memory since 2024. I believe it is solid. >=20 > Why standalone (short version) >=20 > famfs was introduced at LPC 2023 and LSFMM 2024 [1] as a standalone file > system, then spent close to a year being ported to fuse/libfuse. After > maintaining famfs in both forms, I have concluded it makes more sense as = a > standalone file system: fuse adds complexity to famfs (more complex, less > adaptable, less performant - and famfs files are memory, so access must r= un > at memory speed), while famfs adds complexity to fuse that is unlikely to= see > constructive re-use. The full history and argument are in the v12 cover > letter [7]. >=20 > Changes since v12 > ----------------- >=20 > - Patch 01 split, per Alison Schofield: the removal of the now-unused > dax_dev_get() export is a standalone DAX cleanup that goes via the DAX = tree > and is posted separately [8]. Patch 01 of this series is reduced to add= ing > the non-allocating lookup helper dax_dev_find(). >=20 > - Darrick Wong's review comments are addressed across the series: the > kill_*_super() rationale in the mount patch, the page_mkwrite/pfn_mkwri= te > write-fault comment, dropping the operation flags from iomap->flags in = the > resolver, and more. >=20 > - A number of findings from the Sashiko review bot are fixed: > * mprotect() could bypass the FAMFS_OPT_WRITE gate (the check looked = at > VM_WRITE only, not VM_MAYWRITE); > * rename() over an existing target bypassed the FAMFS_OPT_UNLINK / > FAMFS_OPT_RMDIR policy that unlink()/rmdir() enforce; > * splice()/sendfile() returned zeroes on these DAX files > (filemap_splice_read -> copy_splice_read); > * create/mkdir/mknod updated the child inode's timestamps instead of = the > parent directory's; > * .map_pages (filemap_map_pages) was a no-op on DAX and is removed; > * mount-parameter and module-namespace hygiene (reject unknown mount > options; prefix the non-static lookup_daxdev()). > A few Sashiko findings were considered and declined with rationale - fo= r > example generic_write_sync(), which is inert for a filesystem with no p= age > cache, no writeback, and a noop_fsync ->fsync. Those are noted under th= e > relevant patch. >=20 > - famfs now accepts 4 KiB extent alignment in addition to 2 MiB: the > allocation unit may be 4 KiB or 2 MiB. 2 MiB-aligned extents still map = with > huge pages; 4 KiB-granular extents map with PTEs (the DAX fault path al= ready > falls back PMD -> PTE). >=20 > - MAP_SYNC is now supported (FOP_MMAP_SYNC). famfs fmap metadata is immut= able > after MAP_CREATE, so the MAP_SYNC durability guarantee is met with no > metadata sync required. >=20 > - famfs is explicitly 64-bit only now (depends on 64BIT); it targets > CXL/fabric-attached memory, which does not exist on 32-bit systems. >=20 > Famfs Overview >=20 > Famfs exposes sharable disaggregated memory as a file system. Famfs consu= mes > shared memory from [usually shared memory] dax devices, and provides > memory-mappable files that map directly to the memory - no page cache > involvement. Famfs differs from conventional file systems in fs-dax mode, > in that it handles in-memory metadata in a sharable way (which begins wit= h > never caching dirty shared metadata). So a famfs file system can be mount= ed > from multiple nodes, provided they have access to the memory. >=20 > The key performance requirement is that famfs must resolve mapping faults > with minimal overhead. This is achieved by fully caching the file-to-devd= ax > metadata for all active files. >=20 > Famfs remains the first fs-dax file system that is backed by devdax rathe= r > than pmem in fs-dax mode (hence the need for the new dax mode). >=20 > Famfs depends on the 'fsdev' dax mode which landed in 7.1 - it will only = run > with an fsdev-mode (aka famfs-mode) daxdev as its backing device(s). >=20 > The famfs user space can be found at [6]. >=20 > [1] https://lwn.net/Articles/983105/ (Famfs at LSFMM 2024) > [2] https://lwn.net/Articles/1020170/ (Famfs at LSFMM 2025, with patch li= nk) > [3] https://lwn.net/Articles/1068686/ (LWN coverage of the patch thread) > [4] https://lwn.net/Articles/1082687/ (Famfs at LSFMM 2026) > [5] https://lore.kernel.org/linux-fsdevel/0100019f7d9fbe81-6cb16662-2522-= 47ea-a152-fab0ee3d9b35-000000@email.amazonses.com/#b > [6] https://famfs.org > [7] https://lore.kernel.org/linux-fsdevel/20260806053409.GL3560084@frogsf= rogsfrogs/T/#t (v12) > [8] https://lore.kernel.org/linux-cxl/0100019fd478eb93-a6bd48e4-b5f1-4871= -93d4-582802956e6c-000000@email.amazonses.com/T/#u (Partial dax patch) >=20 >=20 > John Groves (12): > dax: replace exported dax_dev_get() with non-allocating dax_dev_find() > famfs: Module operations, fs_context, and mount > famfs: Add daxdev table and dax notify_failure support > famfs: Introduce inode_operations and super_operations > famfs: Introduce file_operations read/write > famfs: Introduce mmap and VM fault handling > famfs: MAP_CREATE ioctl and fmap ingest (ABI 44) > famfs: iomap_begin and file-to-dax offset resolution > famfs: Register secondary daxdevs by path (FAMFSIOC_DAXDEV_OPEN) > famfs: Add runtime operation-permission (opts) framework > famfs: Report device capacity via statfs so df works > famfs: Add documentation >=20 > Documentation/filesystems/famfs.rst | 142 +++ > Documentation/filesystems/index.rst | 1 + > .../userspace-api/ioctl/ioctl-number.rst | 1 + > MAINTAINERS | 8 + > drivers/dax/fsdev.c | 19 + > drivers/dax/super.c | 38 +- > fs/Kconfig | 2 + > fs/Makefile | 1 + > fs/famfs/Kconfig | 12 + > fs/famfs/Makefile | 5 + > fs/famfs/famfs_file.c | 1007 +++++++++++++++++ > fs/famfs/famfs_inode.c | 869 ++++++++++++++ > fs/famfs/famfs_internal.h | 162 +++ > fs/namei.c | 1 + > fs/super.c | 7 + > include/linux/dax.h | 7 +- > include/linux/fs.h | 1 + > include/uapi/linux/famfs_ioctl.h | 160 +++ > include/uapi/linux/magic.h | 1 + > 19 files changed, 2441 insertions(+), 3 deletions(-) > create mode 100644 Documentation/filesystems/famfs.rst > create mode 100644 fs/famfs/Kconfig > create mode 100644 fs/famfs/Makefile > create mode 100644 fs/famfs/famfs_file.c > create mode 100644 fs/famfs/famfs_inode.c > create mode 100644 fs/famfs/famfs_internal.h > create mode 100644 include/uapi/linux/famfs_ioctl.h >=20 > --=20 > 2.53.0 >=20 I looked over the series and while I don't have the greatest grasp of DAX/CXL, the fs bits all look fairly self-contained and reasonable to me. The changes outside of fs/famfs are extremely minimal, so I'm not worried much about this breaking anything unrelated. At this point, I think just taking this as a standalone fs is the right thing to do. This already has field deployments, so it's fairly well tested. If it turns out that the FUSE approach becomes more palatable later, we can just move to deprecate the standalone version then. You and Richard discussed some (minimal) changes in patch #7. Assuming you resolve those, you can add: Reviewed-by: Jeff Layton (but figure that's mostly for the fs and not mm parts). It's a little late for v7.3 given that the merge window is open now. Should we aim to get this into v7.4? That would give this a full cycle in linux-next too.