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id 0A22A601F0; Wed, 21 Oct 2026 14:36:11 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A83FB1F000C1; Wed, 21 Oct 2026 14:36:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792593370; bh=0/DZA0XSq4zWQqR92u51BiCE+QM/y2vLsi5z1/3NCwY=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=V+Mok6ucfzzQtXkUSDemQyWLpiIB8chSnQn2edncNpTvz73tnOPq/gyXgZXCpxc+B 71VEqYOiMs+JDomAwP6farAOge+AM7kKPbiKlnISXHkw+q6eLBItyXfBgh1J2lTjpQ kbCC+zFk7hd/GnUYSY+Ku86LQboRBXGaFXvtEYM0UgydfPfIH4W9hkGgeyIldypEzO vFO62viaHJXrQ2a9kXcKwnNPBjK6l5D7lFj66Zv5kOLNAoTsAR6p6XCBnM0IvZ6Zgw qLqboxHmmnePuG1abV1oVMiLXJba+fyiG6KIxcCJZmVQZCpy6FFOsLSwX4xD9BhltE gZzXBCSBKYKEQ== Message-ID: Subject: Re: [PATCH v7 3/5] mm, iomap: keep a partly covered page until an uncached write completes it 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: Wed, 21 Oct 2026 10:36:07 -0400 In-Reply-To: <20261020220346.28020-4-snitzer@kernel.org> References: <20261020220346.28020-1-snitzer@kernel.org> <20261020220346.28020-4-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 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 USUKUWoG lq+biOcMVqF5iV2Dsw3mxy6kAGCam8gm18JTbQzEMmnZv+oR2n46EgtnisaeY7QW+SkE8FkgSrlllXqv07+IoZkB/1BtDoXfcNkGkRhn9JS2q+eGkbXDwmvUI8v/7d6SSpI5hER0D4d1oqMaUYzuP2tGxXlCbGAzrMTlLSS4jLOyzJJyMmHWFfesc1YcWUekXCbDnwmDK0P7Yp86vcXZTATUebLh1ZR6dLkl5cvg7fFhe8ORO+d96cUGMCc1iruMD7zuI+ifizXpnx777rW2GSUsbsNZYXckgkchCF7BdyTU0Fdvf/MFo05dH/FCisjf5Vbb2 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 Tue, 2026-10-20 at 18:03 -0400, Mike Snitzer wrote: > An uncached (RWF_DONTCACHE) buffered write marks drop-behind every folio > it creates, so writeback drops each one as soon as this write's data is > on disk. A page the write covers only partly is usually shared with the > neighbouring write, the one that ends or starts in it. Once the first > of the two has been written back, the page is gone, and the second has > to read the block back from disk before it can write its part: one read > per misaligned write. >=20 > Measured locally on XFS (emulated 4Kn NVMe, 4 KiB pages), 32 threads > writing interleaved records to one file with pwritev2(), 1 GiB per run, > before and after this patch: >=20 > record flags 4 KiB device MiB/s > bytes reads per write > before after before after > 47008 RWF_DONTCACHE 0.97 0.000 831 2281 > 47008 RWF_DONTCACHE|RWF_DSYNC 0.99 0.000 601 1227 > 6000 RWF_DONTCACHE 0.97 0.000 121 488 > 6000 RWF_DONTCACHE|RWF_DSYNC 0.99 0.000 90 206 >=20 > Before this patch every write waited for its read, and because XFS holds > its IOLOCK exclusively across a buffered write, the other 31 writers > waited too: both record sizes ran at about 20 thousand writes a second, > one device read per write; after it nothing is read back. With > RWF_DSYNC each write still waits for its own sync, so the gain is > smaller: the uncached write now runs as fast as the same write without > RWF_DONTCACHE, 1237 and 209 MiB/s against 1227 and 206 in the table. > Records of 49152 bytes, which cover only whole pages, read nothing > before or after. >=20 > NFSD DIRECT issues the misaligned ends of a WRITE as buffered uncached > segments and gains the same way. Without any handling of the shared > page, 32 interleaved writers of 47008-byte records read 45144 blocks > for 45664 WRITEs; with this patch, and no NFSD code for the shared page, > 0.0001 to 0.0008 blocks per WRITE whether each record is sent as one > FILE_SYNC WRITE, as two one after the other, as two together, or as two > UNSTABLE. The page cache then marks the completed page while it is > still dirty, so each segment of a stable WRITE wakes the DONTCACHE > writeback kick (23 to 65 thousand rounds a run); the next two patches > let NFSD mark it after its sync instead. >=20 > Let the write path say which pages a write covers only partly > (FGP_DONTCACHE_EDGE, set by iomap when the write starts inside a page or > ends before its end) and get such a page as a single page. A folio > created for it is not marked, so writing back this write's part does not > drop it. A folio found uptodate is the neighbour's, already written: > this write completes it, so mark it, under the folio lock and before its > own data goes in, and the writeback that cleans both writes' data drops > it. The folio lock orders the two writes, so the second always finds > the first's data in place. >=20 > A page whose neighbour never writes, such as the last page of a file, > stays cached until reclaim. A partly covered page another task had > cached and that is uptodate is now dropped after the write's writeback, > where before an uncached write never dropped a page it did not create. >=20 > Only iomap file systems say which pages a write covers partly so far; > write_begin_get_folio() users such as ext4 and the NFS client do not. > With blocks smaller than a page, a write that crosses an extent boundary > inside a page asks for it as two partly covered halves, so the page, > though one write covers it whole, stays cached until reclaim. >=20 > Assisted-by: LLM > Signed-off-by: Mike Snitzer > --- > fs/iomap/buffered-io.c | 12 +++++++++++- > include/linux/pagemap.h | 5 +++++ > mm/filemap.c | 19 ++++++++++++++++++- > 3 files changed, 34 insertions(+), 2 deletions(-) >=20 The nfsd case typically uses buffered or DONTCACHE for the unaligned sub-page-sized "ends" when direct is in force, so mostly these ends get submitted as small (< PAGE_SIZE or folio_size()) buffered I/Os. Why do we need this special casing instead of just making those unaligned ends use regular buffered I/O? Isn't it basically the same thing? =20 > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > index 0a5ebfda90f1..780a0186045d 100644 > --- a/fs/iomap/buffered-io.c > +++ b/fs/iomap/buffered-io.c > @@ -802,8 +802,18 @@ struct folio *iomap_get_folio(struct iomap_iter *ite= r, loff_t pos, size_t len) > =20 > if (iter->flags & IOMAP_NOWAIT) > fgp |=3D FGP_NOWAIT; > - if (iter->flags & IOMAP_DONTCACHE) > + if (iter->flags & IOMAP_DONTCACHE) { > fgp |=3D FGP_DONTCACHE; > + /* > + * A page this write covers only partly is shared with a > + * neighbouring write: get it as a single page, which is > + * kept until the write that completes it. > + */ > + if (offset_in_page(pos) || len < PAGE_SIZE) { > + fgp |=3D FGP_DONTCACHE_EDGE; > + len =3D min_t(size_t, len, PAGE_SIZE - offset_in_page(pos)); > + } > + } > fgp |=3D fgf_set_order(len); > =20 > return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT, > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index 3012bd95812a..0c1d6bd09aa0 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -701,6 +701,10 @@ pgoff_t page_cache_prev_miss(struct address_space *m= apping, > * * %FGP_NOWAIT - Don't block on the folio lock. > * * %FGP_STABLE - Wait for the folio to be stable (finished writeback) > * * %FGP_DONTCACHE - Uncached buffered IO > + * * %FGP_DONTCACHE_EDGE - With %FGP_DONTCACHE: the write covers this pa= ge > + * only partly, and a neighbouring write may complete it. A folio cre= ated > + * for it is not marked drop-behind; a folio found uptodate is, as the > + * write that completes it. > * * %FGP_WRITEBEGIN - The flags to use in a filesystem write_begin() > * implementation. > */ > @@ -715,6 +719,7 @@ typedef unsigned int __bitwise fgf_t; > #define FGP_FOR_MMAP ((__force fgf_t)0x00000040) > #define FGP_STABLE ((__force fgf_t)0x00000080) > #define FGP_DONTCACHE ((__force fgf_t)0x00000100) > +#define FGP_DONTCACHE_EDGE ((__force fgf_t)0x00000200) > #define FGF_GET_ORDER(fgf) (((__force unsigned)fgf) >> 26) /* top 6 bits= */ > =20 > #define FGP_WRITEBEGIN (FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE) > diff --git a/mm/filemap.c b/mm/filemap.c > index ed7e4dbea653..0397ec771547 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2016,6 +2016,16 @@ struct folio *__filemap_get_folio_mpol(struct addr= ess_space *mapping, > =20 > if (fgp_flags & FGP_STABLE) > folio_wait_stable(folio); > + > + /* > + * An uncached write that covers this folio only partly, finding it > + * uptodate, is the write that completes it: the neighbouring write > + * that left it here has written its part. Mark it now, under the > + * folio lock, so the writeback that cleans both writes' data drops it. > + */ > + if ((fgp_flags & FGP_DONTCACHE) && (fgp_flags & FGP_DONTCACHE_EDGE) && > + (fgp_flags & FGP_LOCK) && folio_test_uptodate(folio)) > + folio_mark_dropbehind(folio); > no_page: > if (!folio && (fgp_flags & FGP_CREAT)) { > unsigned int min_order =3D mapping_min_folio_order(mapping); > @@ -2053,7 +2063,14 @@ struct folio *__filemap_get_folio_mpol(struct addr= ess_space *mapping, > /* Init accessed so avoid atomic mark_page_accessed later */ > if (fgp_flags & FGP_ACCESSED) > __folio_set_referenced(folio); > - if (fgp_flags & FGP_DONTCACHE) > + /* > + * A folio the write covers only partly is left > + * unmarked, so that writing back this write's part > + * does not drop it before a neighbouring write has > + * written the rest. > + */ > + if ((fgp_flags & FGP_DONTCACHE) && > + !(fgp_flags & FGP_DONTCACHE_EDGE)) > __folio_set_dropbehind(folio); > =20 > err =3D filemap_add_folio(mapping, folio, index, gfp); --=20 Jeff Layton