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506FE433DD; Tue, 27 Oct 2026 14:32:02 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 26B8F1F01825; Tue, 27 Oct 2026 14:32:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793111522; bh=jnV/zbHlczBvT7OWrHzxgEdKY9M+Nd6Op+Y0PO93fvk=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=j05vy9UxbquH0DCLUBNFlbj73LBDWr9DkbMK7QYTqsckETUCGGIWU3G98N/BWFGM6 hkBvIArftZ5eGUGhQCuIRuQGM4EjuI8JQfeyWmRAm2HULYpKfcl4rqwsF9E3Iqkg+L FxaqZyTME6rpbyuVJdvJ6lfRpK4/rfDMGW46qYFBJzFPgBDZa+qyDxkjXjPUfYq6rO LX/PzYp/hDPShAY+6/DOG2ScmnVQXAfhv/3OxqmN1PLWeAJRhZJKptNXTGiX02E8UZ 7Kk7C64ad80sqyJSrEJiN2z4vLRoNAwbNvhldt72/fJABWGEV/6EUD/69yiCZT/1pP 62zbCCWGWGHMA== Message-ID: Subject: Re: [PATCH v9 0/6] mm, iomap, NFSD: stop uncached writes reading back shared pages From: Jeff Layton To: Mike Snitzer , Matthew Wilcox , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Cc: Chuck Lever , linux-nfs@vger.kernel.org, NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Christoph Hellwig , Jens Axboe , Jonathan Corbet Date: Tue, 27 Oct 2026 10:31:58 -0400 In-Reply-To: <20261024221152.6965-1-snitzer@kernel.org> References: <20261024221152.6965-1-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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 U64qk20p 6ilcki5sxTMbs6Yd+M1pCZCiER3bCVO5G9R7TS3zB1k11PmbnGGkxlX4c8pKvv4+CccXfbKT2rzdhJpYKkWgfX6QuU9DKBaQNECxCkJNgsh6t6PtSz0qJNayUn67RM1Mqnrpg9NvbZH2QqmY2/xfrxWsS3BndZXzxy8eV8ZQ4RBp5mC3YT+yGkNntSPj5+7jjw4LrJnRBhcbVNmhYN9dtfH5IXmpb+BjPRIkGNjW2419s8wG57snyWK7karffqVHeE9knO/hp2VJWxY3LLDk4Aboof6DVS+vUhpFOqaxpLu+9Y+jx16yhJqXeXg/Ik1iQ4fHr Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Sat, 2026-10-24 at 18:11 -0400, Mike Snitzer wrote: > Hi, >=20 > Please consider this v9 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. Once the first of the two is > written back, the second reads the page back from disk before it writes > its part. 32 threads wrote interleaved records to one file on > XFS (emulated 4Kn NVMe, 4 KiB pages). Before and after 4/6: >=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 cover only whole > pages, and they 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 > 4/6 has the page cache keep such a page for the write that completes > it. 5/6 lets a writer that syncs its own write complete those pages > itself once the sync is done. NFSD does that for a stable WRITE. 6/6 > is the NFSD change that uses it. 2/6 reports the DONTCACHE dirty count > in the bdi debugfs stats. 3/6 adds folio_mark_dropbehind(), which 4/6 > and 5/6 use. >=20 > 1/6 fixes 88d6f128d06d ("mm: track DONTCACHE dirty pages per > bdi_writeback"), in v7.3. A cached lookup that clears the dropbehind > flag could uncount a folio that was never counted. It could also leave > a folio counted with nothing to uncount it. The patch applies to v7.3 > as it is, if the mm side would rather take it alone. >=20 > v8 1/7 is not in this series. As Chuck asked, I post it on its own. It > is a revised version of 80728157d1bd ("NFSD: cut a misaligned > direct-mode WRITE at page boundaries") in cel/nfsd-testing. This series > applies on cel/nfsd-testing at 6d13b7f74eec ("NFSD: Map every > NFS_ACLv2 reply status into aclstat2"), with or without that revision. > Each patch builds with W=3D1 with no warnings. >=20 > 1/6 to 5/6 are mm, with small iomap changes in 4/6 and 5/6. They 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 v8, from two independent sashiko reviews of v8: the one > Jeff Layton relayed, and the one that sashiko-bot posted to linux-mm. > Some findings were in both. > - 1/6 and 3/6 (both reviews): a folio is counted in WB_DONTCACHE_DIRTY > only when both PG_dirty and its dirty tag are set. v8 tested the tag > alone. __folio_cancel_dirty() clears PG_dirty but leaves the tag, and > so does writeback that stops at EOF after folio_clear_dirty_for_io(). > A folio that stays cached then keeps a stale tag. 1/6 uncounted such > a folio twice, and 3/6 counted it and never uncounted it. > - 4/6, 5/6 and 6/6 (second review): with blocks smaller than the folio, > iomap tracks uptodate per block. The edge that a write completes is > then not uptodate when the write finds it, so it was never marked and > stayed cached until reclaim. This happens with 4 KiB blocks on 16 or > 64 KiB pages. The new folio_dontcache_edge_written() also accepts a > folio whose blocks outside this write are uptodate. > filemap_dontcache_edge_completes() takes the range that the write > covers, and 6/6 passes it. > - 5/6 (second review): filemap_dontcache_edge_complete() now waits for > writeback in flight, which could not drop the folio. It also kicks > DONTCACHE writeback when it marks a dirty folio. The earlier kick > came before the mark. It drops a clean folio that is not mapped > without a mark. A mark that filemap_end_dropbehind() then cleared > outside the i_pages lock could leave WB_DONTCACHE_DIRTY counted. > - 6/6 (both reviews): NFSD finds the edges that a buffered segment > shares by the mapping's smallest folio, as iomap does, not by page. > On 16 KiB blocks, a WRITE written as one buffered segment can have > page-aligned ends inside a block. Nothing completed those edges. > NFSD also completes a segment's first edge only when the write > reached the end of the segment's bytes in that unit (first review). > - 4/6 (first review): a paragraph on a limit. 4/6 marks the folio > before iomap validates the iomap. A stale iomap stopped by a fatal > signal then leaves a mark on a folio that this write did not complete. > This costs one read-back and no data. > - 2/6: unchanged. >=20 > Patches 1, 3, 4, 5 and 6 carry notes below the --- line. The notes > give each question from the v8 reviews, and the code, lock or check > that answers it. This includes the limits that the series keeps on > purpose. >=20 > Before posting, I ran a third review of v9 locally, with Sashiko's > review stages and the review prompts it uses. It found no new > problems. >=20 > Measured for 6/6 on 16 KiB-block XFS over the loop device. Its > dma_alignment is 511, so every TCP payload is memory-misaligned and > NFSD writes each WRITE as one buffered segment. 32 writers of 12288-byte > records, 1 GiB: v8 left all 32,736 pages of a 128 MiB file resident. > v9 leaves 948 of 262,080 pages (0.4%) and reads 0.0003 blocks per > record. The v8 results hold. In every run BdiDontcacheDirty was 0 > after the final sync. That includes runs that mix uncached writes, > cached writes and mmap stores on one file. Every record verified. >=20 > I measured all of v9 again on 4 KiB pages, with XFS blocks of 1 KiB > and 2 KiB over the loop device. These blocks are smaller than the page. > 32 writers wrote 47008-byte records, 1 GiB. Uncached application writes > left 0 of 261,850 pages resident. NFSD left 497 (1 KiB blocks) and 349 > (2 KiB blocks). LOCALIO left 854 and 949. Every path read 0.0001 > blocks per record. A kernel without these fixes, on a 128 MiB run with > 1 KiB blocks, left about one page for every two records. Uncached > writes through NFSD, cached writes and mmap stores ran together on one > file, with 4 KiB and 1 KiB blocks. BdiDontcacheDirty was 0 after every > round, and every record verified. I did not test kernels with 16 KiB > or 64 KiB pages. >=20 > v8: https://lore.kernel.org/linux-nfs/20261023024548.71123-1-snitzer@kern= el.org/ > 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 (6): > 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 >=20 > .../filesystems/nfs/nfsd-io-modes.rst | 17 ++ > fs/iomap/buffered-io.c | 36 ++- > fs/iomap/trace.h | 3 +- > fs/nfsd/vfs.c | 116 +++++++- > include/linux/fs.h | 8 +- > include/linux/iomap.h | 1 + > include/linux/pagemap.h | 14 + > mm/backing-dev.c | 6 + > mm/filemap.c | 265 +++++++++++++++++- > 9 files changed, 435 insertions(+), 31 deletions(-) >=20 >=20 > base-commit: 6d13b7f74eec16e08d5e7f772ee119432a14ef2c This all looks good to me at this point. This is a mixed mm and nfsd set though. How do you envision this being merged? Reviewed-by: Jeff Layton