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header.b=clD++HFp Received: by smtp.kernel.org (Postfix) with ESMTPSA id E649F1F01825; Fri, 23 Oct 2026 10:57:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792753033; bh=AZy3vSXQuFsVXgDu4mp0Kw/AOG663vFWNbTt1QMJvQo=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=clD++HFpIMJt5lFVSwNArhp2/vBs1rRNdXKpSeyrbn2PGQCodef/gW7FnXsNXcipR sb8xMFQ1PSwA2V6OuRZbg2rlDpqj1a06L8n/etzVQWUngdD7VCS5F8g1MS2Pj6hTQs tn2bhz0DUoNpVyqGdWjwujA1C8N09H4spcItVHRK/cJFm7W9VxW5q2GxFOXq1w2+JQ ZXEcs64hmuB5/TYhO7UyshFWiObRjNqgYngEFrlzV4jf5QxmapQ5zhzCv7+w1oaAki L6ks2VOWkTti73T9soedlZHX3Rxe9VNqT7oS4r6AVsy0XsdKTxc+YTzGnhmbYJMVN1 E3eqpeHwVW81w== Message-ID: <9b43a1aec170ee810ef5fd2a2ab1288bad6508fd.camel@kernel.org> Subject: Re: [PATCH v8 0/7] mm, iomap, NFSD: stop uncached writes reading back shared pages From: Jeff Layton To: Mike Snitzer , Chuck Lever Cc: linux-nfs@vger.kernel.org, NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Christoph Hellwig , Jens Axboe , Matthew Wilcox , Jonathan Corbet , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Date: Fri, 23 Oct 2026 06:57:09 -0400 In-Reply-To: <20261023024548.71123-1-snitzer@kernel.org> References: <20261023024548.71123-1-snitzer@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf CBYyUE8QJpvTZyfUHNYldXlrjO6n5MdOempLqWpfOmcGkwnyNRBR46g/jf8KnPRwXs509yAqDB6sE 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AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-2.fc44) X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Thu, 2026-10-22 at 22:45 -0400, Mike Snitzer wrote: > Hi, >=20 > Please consider this v8 for the v7.5 development cycle, so that it can > go upstream in the v7.5 merge window. >=20 > An uncached (RWF_DONTCACHE) buffered write drops each page it writes > once that page is written back. A page the write covers only partly is > shared with the neighbouring write, and once the first of the two has > been written back the second has to read it back from disk before it > can write its part. 32 threads writing interleaved records to one file > on XFS (emulated 4Kn NVMe, 4 KiB pages), before and after 5/7: >=20 > record pwritev2() flags 4 KiB device MiB/s > bytes reads per write > before after before after > 47008 RWF_DONTCACHE 0.97 0.0005 831 2281 > 47008 RWF_DONTCACHE|RWF_DSYNC 0.99 0.0005 601 1227 > 6000 RWF_DONTCACHE 0.97 0.0001 121 488 > 6000 RWF_DONTCACHE|RWF_DSYNC 0.99 0.0002 90 206 >=20 > What is left after is metadata: 49152-byte records, which cover only > whole pages, read 0.0005 per write either way. The test program and > script that produced this table, and how to run them, are in: > https://lore.kernel.org/linux-nfs/atqJBS8MlYfeUU-t@kernel.org/ >=20 > 5/7 has the page cache keep such a page for the write that completes > it. 6/7 lets a writer that syncs its own write, as NFSD does for a > stable WRITE, complete those pages itself once the sync is done. 7/7 > is the NFSD change that uses it. 3/7 reports the DONTCACHE dirty count > in the bdi debugfs stats, and 4/7 adds folio_mark_dropbehind(), which > 5/7 and 6/7 use. >=20 > 1/7 and 2/7 are fixes the review of v7 turned up in code that is > already queued or upstream, and each stands on its own: >=20 > - 1/7 fixes 80728157d1bd ("NFSD: cut a misaligned direct-mode WRITE at > page boundaries") in cel/nfsd-testing: where the mapping's smallest > folio is a block larger than a page, NFSD cut a WRITE's misaligned > ends inside a block, so the direct middle and a buffered end shared > it. Chuck, maybe best to fold into 80728157d1bd ? > - 2/7 fixes 88d6f128d06d ("mm: track DONTCACHE dirty pages per > bdi_writeback"), in v7.3: a cached lookup that clears the dropbehind > flag could take a folio off WB_DONTCACHE_DIRTY that was never counted > there, or leave one counted with nothing to uncount it. It applies > to v7.3 as it is, if the mm side would rather take it alone. >=20 > The prerequisite NFSD changes are staged in cel/nfsd-testing; this > series applies on top of it at 6d13b7f74eec ("NFSD: Map every NFS_ACLv2 > reply status into aclstat2"), and each patch builds with W=3D1 with no > warnings. >=20 > 2/7 to 6/7 are mm, with small iomap changes in 5/7 and 6/7, and are > Cc'd to linux-mm and linux-fsdevel. If the mm and iomap maintainers > are agreeable, their acks would let Chuck take the series through the > nfsd tree. But whatever approach the maintainers prefer for this kind > of cross-subsystem merge is obviously fine. >=20 > Changes since v7, from Jeff's review and the sashiko findings he > relayed: > - 1/7 and 2/7 are new (above). > - 3/7 (v7 1/5): a comment dropped; Jeff's Reviewed-by. > - 4/7 (v7 2/5): folio_mark_dropbehind() counted a folio as DONTCACHE > dirty when it found PG_dirty set, but PG_dirty is set before > folio_account_dirtied() counts the folio, without the folio lock, so > a folio marked in between was counted twice. It now sets the flag > and tests the dirty tag, which is set in the same i_pages section that > counts the folio, under the i_pages lock. > - 5/7 (v7 3/5): the edge is the smallest folio the mapping allows, a > page or a block where blocks are larger, both in iomap's test and in > what is marked: a larger folio found there, left by readahead or > another buffered write, is no longer marked whole. > - 6/7 (v7 4/5): an edge already clean is dropped before the folio lock > is released, not after with a trylock that could miss it; and only > an edge-sized folio, as in 5/7. > - 7/7 (v7 5/5): unchanged. >=20 > Tested on XFS on an emulated 4Kn NVMe, 4 KiB pages, 32 threads of > interleaved 47008-byte records (12288 bytes where noted), 4 KiB device > reads per write, v7 against v8: >=20 > v7 v8 > RWF_DONTCACHE over large folios > readahead left 0.94 0.0026 > RWF_DONTCACHE, 16 KiB blocks 1.62 0.0014 > 12288-byte records 2.89 0.0004 > NFS WRITEs through NFSD DIRECT, > 16 KiB blocks 3.46 0.0016 >=20 > (v7 in 128 MiB runs, v8 in 1 GiB runs; on 16 KiB blocks one block read > back per write is 4.0 here.) In every run of v8, BdiDontcacheDirty > (3/7) was back at 0 after the final sync, including runs mixing > uncached writes, cached writes and mmap stores on one file, and every > record verified. >=20 > v7: https://lore.kernel.org/linux-nfs/20261020220346.28020-1-snitzer@kern= el.org/ > v6: https://lore.kernel.org/linux-nfs/atbfEbgMU6UGkn2X@kernel.org/ > v5: https://lore.kernel.org/linux-nfs/20261019150725.1507-1-snitzer@kerne= l.org/ > v4: https://lore.kernel.org/linux-nfs/20261015233309.42999-1-snitzer@kern= el.org/ > v3: https://lore.kernel.org/linux-nfs/20261001045502.48381-1-snitzer@kern= el.org/ > v2: https://lore.kernel.org/linux-nfs/20260929231329.22018-1-snitzer@kern= el.org/ > v1: https://lore.kernel.org/linux-nfs/20260929173423.16149-1-snitzer@kern= el.org/ >=20 > Thanks, > Mike >=20 > Mike Snitzer (7): > NFSD: cut a misaligned direct-mode WRITE at the mapping's smallest foli= o > mm: take a folio off WB_DONTCACHE_DIRTY only if it was counted there > mm: expose WB_DONTCACHE_DIRTY in bdi debugfs stats > mm: add folio_mark_dropbehind() to mark a folio dropbehind and account = it > mm, iomap: keep a partly covered page until an uncached write completes= it > mm, iomap: let an uncached writer complete its shared pages itself > NFSD: drop a shared boundary page only once both WRITEs have written it >=20 > .../filesystems/nfs/nfsd-io-modes.rst | 17 ++ > fs/iomap/buffered-io.c | 20 +- > fs/iomap/trace.h | 3 +- > fs/nfsd/vfs.c | 113 ++++++++-- > include/linux/fs.h | 8 +- > include/linux/iomap.h | 1 + > include/linux/pagemap.h | 12 ++ > mm/backing-dev.c | 6 + > mm/filemap.c | 193 ++++++++++++++++-- > 9 files changed, 338 insertions(+), 35 deletions(-) >=20 >=20 > base-commit: 6d13b7f74eec16e08d5e7f772ee119432a14ef2c It might not hurt to have your LLM simplify the patch descriptions here. I find that my appetite for reading claude-ish walls of text has waned in recent weeks. This skill helps a lot: https://github.com/danyuchn/asd-ste100-skill --=20 Jeff Layton