From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792580067; b=OugJI6m13xhmnkbhhzKSNX9rslsqSV0i1mZZo6TE5bWSXF7e/a/vrzbPTCUbx0IgcQIV Yim47yAruIh0gK+IpVykLsCcKIgjxNtGl2ud9GQ8AZoGverE3tEyKucT8aST+mjTg1MX8 +Ju4HR80rTAF2+a29cWUDe1dOHEQ1WSpRRoFh59oAcEgGKMrnuXkCXqBfdfc7BlyN3bTt pG3yQfSYVgKbCHd7dGDJZH35rr6uOQ64AkweQHyk3mJGjdH2lOnrdtcbMyKDQWHLAgRnt S1wPGU1cmWtbNE1WJOLato1wc+4dgwwEubJDpxCzxu1LlJoQHouE0v7/s0ZyAKB0f4g== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792580067; h=Received:DKIM-Signature:Message-ID:Subject:From:To:Cc:Date: In-Reply-To:References:Autocrypt:Content-Type: Content-Transfer-Encoding:User-Agent:MIME-Version; bh=zCYVniZFly3PgMtnN2QXPJZFmZMEuBDbCk5iztknPcQ=; b=1cPTSKAleihaQU49OUzyEy6naPpysu/J1e6yfogmJ5DhSM7Yp7EoMoeuJoJVQtwZ5HiI 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header.b=VqkODNi8 Received: by smtp.kernel.org (Postfix) with ESMTPSA id BA3CF1F000CA; Wed, 21 Oct 2026 10:54:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792580066; bh=zCYVniZFly3PgMtnN2QXPJZFmZMEuBDbCk5iztknPcQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=VqkODNi8gJ70pwQfR6D9EDRyyxh5axW1MkIbPK59SPJng2jOcAaDxxj6UidmBWK8y Ran5QHkny0jU2tYLwBvYSIYMmygsM0M3ewvMd6KQ9OdPTPfhCQPaPk00K65yybWs69 tsEzAtdcMFEX4W3LMK9kExNfLKDGbLiGX04x6aUoJfZNdzjXbGUf4a8r+6nT8f8euH G57xqiGVa197xfKUd23Ds+1mZ4aWvdzrSLoRD7kU/kDD/3vWPJIhEIqRhRvKjz0M2f texQDr/1r48IjnfkpPRE2UekhbdhrNuLrDtthl8mEToqcNNAihqZRkUYqycCKATyZv O+8wuEk1gOWRg== Message-ID: <39b14c2a978ec71138492debe95d883aebc3cfe5.camel@kernel.org> Subject: Re: [PATCH v7 4/5] mm, iomap: let an uncached writer complete its shared pages itself 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 06:54:14 -0400 In-Reply-To: <20261020220346.28020-5-snitzer@kernel.org> References: <20261020220346.28020-1-snitzer@kernel.org> <20261020220346.28020-5-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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Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV 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 Tue, 2026-10-20 at 18:03 -0400, Mike Snitzer wrote: > The write that completes a page an uncached write covers only partly > marks it drop-behind as it writes, while the page is dirty, so the page > counts as DONTCACHE dirty and the DONTCACHE writeback kick writes it > back. A writer that syncs its write itself, once, after several > segments -- NFSD answering a stable WRITE, the NFS client's LOCALIO path > -- issues each segment without IOCB_DSYNC, so every segment's kick > writes back pages its own sync is about to write. Measured with NFSD > DIRECT and 32 writers of 47008-byte records: with the previous patch > alone, 23,000 to 65,000 kick rounds a run, where NFSD marking the pages > itself after its sync had taken 51 to 274; with this patch and NFSD > using it, a FILE_SYNC WRITE wakes the kick not at all. >=20 > Let such a writer complete those pages itself. IOCB_DONTCACHE_DEFER > (IOMAP_DONTCACHE_DEFER, FGP_DONTCACHE_DEFER) leaves a partly covered page > unmarked even when the write finds it uptodate. Right before writing suc= h > a page the writer asks filemap_dontcache_edge_completes() whether it is > uptodate already, which means the neighbouring write has written its part > and this write completes it; once the write and its sync are done, > filemap_dontcache_edge_complete() marks it and, clean, drops it at once. >=20 > Two writes that arrive together may both find the page absent before > either has written; that page is then left cached until reclaim, which > the folio-lock ordering of the default path avoids. >=20 > Assisted-by: LLM > Signed-off-by: Mike Snitzer > --- > fs/iomap/buffered-io.c | 4 +++ > fs/iomap/trace.h | 3 ++- > include/linux/fs.h | 8 +++++- > include/linux/iomap.h | 1 + > include/linux/pagemap.h | 6 +++++ > mm/filemap.c | 58 ++++++++++++++++++++++++++++++++++++++++- > 6 files changed, 77 insertions(+), 3 deletions(-) >=20 > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > index 780a0186045d..5ca82000e2ec 100644 > --- a/fs/iomap/buffered-io.c > +++ b/fs/iomap/buffered-io.c > @@ -811,6 +811,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter= , loff_t pos, size_t len) > */ > if (offset_in_page(pos) || len < PAGE_SIZE) { > fgp |=3D FGP_DONTCACHE_EDGE; > + if (iter->flags & IOMAP_DONTCACHE_DEFER) > + fgp |=3D FGP_DONTCACHE_DEFER; > len =3D min_t(size_t, len, PAGE_SIZE - offset_in_page(pos)); > } > } > @@ -1318,6 +1320,8 @@ iomap_file_buffered_write(struct kiocb *iocb, struc= t iov_iter *i, > iter.flags |=3D IOMAP_NOWAIT; > if (iocb->ki_flags & IOCB_DONTCACHE) > iter.flags |=3D IOMAP_DONTCACHE; > + if (iocb->ki_flags & IOCB_DONTCACHE_DEFER) > + iter.flags |=3D IOMAP_DONTCACHE_DEFER; > =20 > while ((ret =3D iomap_iter(&iter, ops)) > 0) > iter.status =3D iomap_write_iter(&iter, i, write_ops); > diff --git a/fs/iomap/trace.h b/fs/iomap/trace.h > index e1ea8392cf47..4f0343eeed1b 100644 > --- a/fs/iomap/trace.h > +++ b/fs/iomap/trace.h > @@ -104,7 +104,8 @@ DEFINE_RANGE_EVENT(iomap_zero_iter); > { IOMAP_UNSHARE, "UNSHARE" }, \ > { IOMAP_DAX, "DAX" }, \ > { IOMAP_ATOMIC, "ATOMIC" }, \ > - { IOMAP_DONTCACHE, "DONTCACHE" } > + { IOMAP_DONTCACHE, "DONTCACHE" }, \ > + { IOMAP_DONTCACHE_DEFER, "DONTCACHE_DEFER" } > =20 > #define IOMAP_F_FLAGS_STRINGS \ > { IOMAP_F_NEW, "NEW" }, \ > diff --git a/include/linux/fs.h b/include/linux/fs.h > index f9d1e05e8ae6..ad181b6ce6b4 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -357,6 +357,11 @@ struct readahead_control; > /* kiocb is a read or write operation submitted by fs/aio.c. */ > #define IOCB_AIO_RW (1 << 22) > #define IOCB_HAS_METADATA (1 << 23) > +/* > + * With IOCB_DONTCACHE: the caller completes the pages the write covers = only > + * partly itself, see filemap_dontcache_edge_complete(). > + */ > +#define IOCB_DONTCACHE_DEFER (1 << 24) > =20 > /* for use in trace events */ > #define TRACE_IOCB_STRINGS \ > @@ -374,7 +379,8 @@ struct readahead_control; > { IOCB_NOIO, "NOIO" }, \ > { IOCB_ALLOC_CACHE, "ALLOC_CACHE" }, \ > { IOCB_AIO_RW, "AIO_RW" }, \ > - { IOCB_HAS_METADATA, "AIO_HAS_METADATA" } > + { IOCB_HAS_METADATA, "AIO_HAS_METADATA" }, \ > + { IOCB_DONTCACHE_DEFER, "DONTCACHE_DEFER" } > =20 > struct kiocb { > struct file *ki_filp; > diff --git a/include/linux/iomap.h b/include/linux/iomap.h > index bc7ae6327dbf..f3509d495a92 100644 > --- a/include/linux/iomap.h > +++ b/include/linux/iomap.h > @@ -212,6 +212,7 @@ struct iomap_write_ops { > #endif /* CONFIG_FS_DAX */ > #define IOMAP_ATOMIC (1 << 9) /* torn-write protection */ > #define IOMAP_DONTCACHE (1 << 10) > +#define IOMAP_DONTCACHE_DEFER (1 << 11) /* see IOCB_DONTCACHE_DEFER */ > =20 > /* > * Return the existing mapping at pos, or reserve space starting at pos = for up > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h > index 0c1d6bd09aa0..a0a67e4c940b 100644 > --- a/include/linux/pagemap.h > +++ b/include/linux/pagemap.h > @@ -705,6 +705,9 @@ pgoff_t page_cache_prev_miss(struct address_space *ma= pping, > * 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_DONTCACHE_DEFER - With %FGP_DONTCACHE_EDGE: the caller complet= es the > + * page itself (filemap_dontcache_edge_complete()), so a folio found u= ptodate > + * is not marked either. > * * %FGP_WRITEBEGIN - The flags to use in a filesystem write_begin() > * implementation. > */ > @@ -720,6 +723,7 @@ typedef unsigned int __bitwise fgf_t; > #define FGP_STABLE ((__force fgf_t)0x00000080) > #define FGP_DONTCACHE ((__force fgf_t)0x00000100) > #define FGP_DONTCACHE_EDGE ((__force fgf_t)0x00000200) > +#define FGP_DONTCACHE_DEFER ((__force fgf_t)0x00000400) > #define FGF_GET_ORDER(fgf) (((__force unsigned)fgf) >> 26) /* top 6 bits= */ > =20 > #define FGP_WRITEBEGIN (FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE) > @@ -1316,6 +1320,8 @@ void folio_end_writeback(struct folio *folio); > void folio_end_writeback_no_dropbehind(struct folio *folio); > void folio_end_dropbehind(struct folio *folio); > void folio_mark_dropbehind(struct folio *folio); > +bool filemap_dontcache_edge_completes(struct address_space *mapping, lof= f_t pos); > +void filemap_dontcache_edge_complete(struct address_space *mapping, loff= _t pos); > void folio_wait_stable(struct folio *folio); > void __folio_mark_dirty(struct folio *folio, struct address_space *, int= warn); > void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb= ); > diff --git a/mm/filemap.c b/mm/filemap.c > index 0397ec771547..7d7c56a42643 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -1675,6 +1675,61 @@ void folio_mark_dropbehind(struct folio *folio) > } > EXPORT_SYMBOL_GPL(folio_mark_dropbehind); > =20 > +/** > + * filemap_dontcache_edge_completes - will this write complete a shared = page? > + * @mapping: the file's mapping > + * @pos: any byte offset within the page > + * > + * For an uncached writer that completes the pages it covers only partly > + * itself (IOCB_DONTCACHE_DEFER). Call right before writing such a page= : it > + * is in the page cache and uptodate when the neighbouring write has alr= eady > + * written its part, and this write then completes it. Two writes that > + * arrive together may both find it absent; the page is then left cached= , > + * unmarked, until reclaim. > + * > + * Return: true if the caller is to call filemap_dontcache_edge_complete= () > + * once its write, including any sync of it, is done. > + */ > +bool filemap_dontcache_edge_completes(struct address_space *mapping, lof= f_t pos) > +{ > + struct folio *folio; > + bool ret; > + > + /* FGP_DONTCACHE: a plain lookup would clear a drop-behind mark. */ > + folio =3D __filemap_get_folio(mapping, pos >> PAGE_SHIFT, FGP_DONTCACHE= , 0); > + if (IS_ERR(folio)) > + return false; > + ret =3D folio_test_uptodate(folio); > + folio_put(folio); > + return ret; > +} > +EXPORT_SYMBOL_GPL(filemap_dontcache_edge_completes); > + > +/** > + * filemap_dontcache_edge_complete - drop a shared page both writes have= written > + * @mapping: the file's mapping > + * @pos: any byte offset within the page > + * > + * Mark the page drop-behind. If it is already written back, drop it no= w; > + * otherwise the writeback that cleans it drops it. A dirty page marked= here > + * counts as DONTCACHE dirty and wakes the DONTCACHE writeback kick, so = a > + * caller that syncs its write calls this after the sync. > + */ > +void filemap_dontcache_edge_complete(struct address_space *mapping, loff= _t pos) > +{ > + struct folio *folio; > + > + folio =3D __filemap_get_folio(mapping, pos >> PAGE_SHIFT, > + FGP_LOCK | FGP_DONTCACHE, 0); > + if (IS_ERR(folio)) > + return; > + folio_mark_dropbehind(folio); > + folio_unlock(folio); > + folio_end_dropbehind(folio); > + folio_put(folio); > +} > +EXPORT_SYMBOL_GPL(filemap_dontcache_edge_complete); > + > /** > * folio_end_writeback_no_dropbehind - End writeback against a folio. > * @folio: The folio. > @@ -2024,7 +2079,8 @@ struct folio *__filemap_get_folio_mpol(struct addre= ss_space *mapping, > * 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)) > + !(fgp_flags & FGP_DONTCACHE_DEFER) && (fgp_flags & FGP_LOCK) && > + folio_test_uptodate(folio)) > folio_mark_dropbehind(folio); > no_page: > if (!folio && (fgp_flags & FGP_CREAT)) { 2 comments from Chuck's sashiko on this one: [Severity: Low] Could filemap_dontcache_edge_complete() in mm/filemap.c avoid missing the immediate drop when another task acquires the folio lock between folio_unlock() and folio_end_dropbehind()? In that case folio_end_dropbehin= d() fails its folio_trylock() and does not retry; if the folio is already clean= , there may be no later writeback completion to drop it, so it can remain cac= hed until unrelated activity or reclaim. [Severity: Medium] Could filemap_dontcache_edge_complete() in mm/filemap.c restrict marking to the boundary page when the mapping returns a large folio? iomap_get_folio() in fs/iomap/buffered-io.c limits a partial edge request to one page, but th= e mapping's minimum folio order can still cause __filemap_get_folio_mpol() to return a multi-page folio, or the lookup can find an existing large folio. This helper then marks the entire folio for drop-behind. For example, if a caller uses deferred completion on an XFS mapping with a minimum folio orde= r, marking one edge can account the whole dirty folio for DONTCACHE writeback = or invalidate the whole clean, unmapped folio. Could this path handle only the page covered by the completed edge? --=20 Jeff Layton