From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 C9046347516; Thu, 26 Mar 2026 15:24:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774538641; cv=none; b=ZDah0AseTuJlo/AvWlelgqNK8DMG3MLIf7vOOLzo7RZ7FqQo3b3aR2drGZQ3mroaNwAQpEWF7PkBg8nENUpX85xCODKx5Wb3zpM7DxpfTdno0c2H1rE+0R8rxSLylmm7YkksEtzlAG8fixNqO89IjUTgwKH3mKi+PnNQeSA4aio= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774538641; c=relaxed/simple; bh=uzBxhpARfo+cByqFAr4qtGU7seLYatmUFHSkZ/t2D4c=; h=Message-ID:Subject:From:To:Cc:Date:Content-Type:MIME-Version; b=iIoaK2Be01NcY9lS4J7ETcWZRheYmPhmAzPLtxaMni6JI75oqs3YdmCzQnoAP+jl6lmonT4+JKqlICgPjs0gURooheLBZJPND7wPPNd2/7p3/j6DJt33f8+tOTb6Vbnw5gKyYShACHyGSJldhhhqjBfo1H/z6MvNjmVXZ7DeaLQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ASd2As1e; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ASd2As1e" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2241C116C6; Thu, 26 Mar 2026 15:24:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774538641; bh=uzBxhpARfo+cByqFAr4qtGU7seLYatmUFHSkZ/t2D4c=; h=Subject:From:To:Cc:Date:From; b=ASd2As1egjr08oxav7A34W0TRAaalHYlnazzzydkAJaQD9NYtChjid1b+l+DqMPzC agmkdXqeUdOxtfTv2TG+Vw588PBqObVc1U78jDt3F94ccBLnSXY381wy8euw3Ue1oe K0NJp4wo8qC+w81AI2mM/ItqHUVTmfKub4sNbA0K7wq3Lv+hf9dhKn6YoydNC+zt4H 3+6AZYnNvahwOKkGU0XbRMFh85zI1cExUqyAi30LGGd4yH81h09OmQiXRyawlIijX7 Z9YH51/JKAYawd3xBmyWzteL9npsbGiMGGhGzMNjN70OTLI4TgcPMCeRfDqdQOcyFL BUd/0dfjkgn/w== Message-ID: <4ebbd194ccfb3bcea6225d926b4c9f339e21c813.camel@kernel.org> Subject: A comparison of the new nfsd iomodes (and an experimental one) From: Jeff Layton To: linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org Cc: Chuck Lever , Mike Snitzer , Jens Axboe Date: Thu, 26 Mar 2026 11:23:58 -0400 Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV 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The results have been interesting, but one thing that stands out is that RWF_DONTCACHE is absolutely terrible for streaming write workloads. That prompted me to experiment with a new iomode that added some optimizations (DONTCACHE_LAZY). The results along with Claude's analysis are here: https://markdownpastebin.com/?id=3D387375d00b5443b3a2e37d58a062331f He gets a bit out over his skis on the upstream plan, but tl;dr is that DONTCACHE_LAZY (which is DONTCACHE with some optimizations) outperforms the other write iomodes. The core DONTCACHE_LAZY patch is below. I doubt we'll want a new iomode long-term. What we'll probably want to do is modify DONTCACHE to work like DONTCACHE_LAZY: -------------------8<------------------- [PATCH] mm: add IOCB_DONTCACHE_LAZY and RWF_DONTCACHE_LAZY IOCB_DONTCACHE flushes all dirty pages on every write via filemap_flush_range() with nr_to_write=3DLONG_MAX. Under concurrent writers, this creates severe serialization: every writer contends on the writeback submission path, leading to catastrophic throughput collapse (~1 GB/s vs ~10 GB/s for buffered) and multi-second tail latency. Add IOCB_DONTCACHE_LAZY as a gentler alternative with two mechanisms: 1. Skip-if-busy: check mapping_tagged(PAGECACHE_TAG_WRITEBACK) before flushing. If writeback is already in progress on the mapping, the flush is skipped entirely, eliminating writeback submission contention between concurrent writers. 2. Proportional cap: when flushing does occur, cap nr_to_write to the number of pages just written. This prevents any single write from triggering a full-file flush that would starve concurrent readers. Together these mechanisms rate-limit writeback to match the incoming write rate while avoiding I/O bursts that cause tail latency spikes. Like IOCB_DONTCACHE, pages touched under IOCB_DONTCACHE_LAZY are marked for eviction (dropbehind) to keep page cache usage bounded. Also add RWF_DONTCACHE_LAZY (0x200) as a user-visible pwritev2/io_uring flag that maps to IOCB_DONTCACHE_LAZY. The flag follows the same validation as RWF_DONTCACHE: the filesystem must support FOP_DONTCACHE, DAX is not supported, and RWF_DONTCACHE and RWF_DONTCACHE_LAZY are mutually exclusive. Signed-off-by: Jeff Layton --- fs/iomap/buffered-io.c | 2 +- include/linux/fs.h | 18 ++++++++++++++++-- include/linux/pagemap.h | 2 +- include/uapi/linux/fs.h | 6 +++++- mm/filemap.c | 40 +++++++++++++++++++++++++++++++++++++--- 5 files changed, 60 insertions(+), 8 deletions(-) diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index e3bedcbb5f1ea..069d4378bf457 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -1185,7 +1185,7 @@ iomap_file_buffered_write(struct kiocb *iocb, struct = iov_iter *i, =20 if (iocb->ki_flags & IOCB_NOWAIT) iter.flags |=3D IOMAP_NOWAIT; - if (iocb->ki_flags & IOCB_DONTCACHE) + if (iocb->ki_flags & (IOCB_DONTCACHE | IOCB_DONTCACHE_LAZY)) iter.flags |=3D IOMAP_DONTCACHE; =20 while ((ret =3D iomap_iter(&iter, ops)) > 0) diff --git a/include/linux/fs.h b/include/linux/fs.h index 94695ce5e25b5..04ff531473e82 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -359,6 +359,7 @@ 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) +#define IOCB_DONTCACHE_LAZY (__force int) RWF_DONTCACHE_LAZY =20 /* for use in trace events */ #define TRACE_IOCB_STRINGS \ @@ -376,7 +377,8 @@ 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_LAZY, "DONTCACHE_LAZY" } =20 struct kiocb { struct file *ki_filp; @@ -2589,6 +2591,8 @@ extern int __must_check file_write_and_wait_range(str= uct file *file, loff_t start, loff_t end); int filemap_flush_range(struct address_space *mapping, loff_t start, loff_t end); +int filemap_dontcache_writeback_range(struct address_space *mapping, + loff_t start, loff_t end, ssize_t nr_written); =20 static inline int file_write_and_wait(struct file *file) { @@ -2626,6 +2630,12 @@ static inline ssize_t generic_write_sync(struct kioc= b *iocb, ssize_t count) =20 filemap_flush_range(mapping, iocb->ki_pos - count, iocb->ki_pos - 1); + } else if (iocb->ki_flags & IOCB_DONTCACHE_LAZY) { + struct address_space *mapping =3D iocb->ki_filp->f_mapping; + + filemap_dontcache_writeback_range(mapping, + iocb->ki_pos - count, + iocb->ki_pos - 1, count); } =20 return count; @@ -3393,13 +3403,17 @@ static inline int kiocb_set_rw_flags(struct kiocb *= ki, rwf_t flags, if (!(ki->ki_filp->f_mode & FMODE_CAN_ATOMIC_WRITE)) return -EOPNOTSUPP; } - if (flags & RWF_DONTCACHE) { + if (flags & (RWF_DONTCACHE | RWF_DONTCACHE_LAZY)) { /* file system must support it */ if (!(ki->ki_filp->f_op->fop_flags & FOP_DONTCACHE)) return -EOPNOTSUPP; /* DAX mappings not supported */ if (IS_DAX(ki->ki_filp->f_mapping->host)) return -EOPNOTSUPP; + /* can't use both at once */ + if ((flags & (RWF_DONTCACHE | RWF_DONTCACHE_LAZY)) =3D=3D + (RWF_DONTCACHE | RWF_DONTCACHE_LAZY)) + return -EINVAL; } kiocb_flags |=3D (__force int) (flags & RWF_SUPPORTED); if (flags & RWF_SYNC) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 9f5c4e8b4a7d3..3539a7b4ed53c 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -798,7 +798,7 @@ static inline struct folio *write_begin_get_folio(const= struct kiocb *iocb, =20 fgp_flags |=3D fgf_set_order(len); =20 - if (iocb && iocb->ki_flags & IOCB_DONTCACHE) + if (iocb && iocb->ki_flags & (IOCB_DONTCACHE | IOCB_DONTCACHE_LAZY= )) fgp_flags |=3D FGP_DONTCACHE; =20 return __filemap_get_folio(mapping, index, fgp_flags, diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h index 66ca526cf786c..74f7c75901e0c 100644 --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h @@ -434,10 +434,14 @@ typedef int __bitwise __kernel_rwf_t; /* prevent pipe and socket writes from raising SIGPIPE */ #define RWF_NOSIGNAL ((__force __kernel_rwf_t)0x00000100) =20 +/* buffered IO that drops the cache after reading or writing data, + * with rate-limited writeback (skip if writeback already in progress) */ +#define RWF_DONTCACHE_LAZY ((__force __kernel_rwf_t)0x00000200) + /* mask of flags supported by the kernel */ #define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\ RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC |\ - RWF_DONTCACHE | RWF_NOSIGNAL) + RWF_DONTCACHE | RWF_NOSIGNAL | RWF_DONTCACHE_LAZY) =20 #define PROCFS_IOCTL_MAGIC 'f' =20 diff --git a/mm/filemap.c b/mm/filemap.c index 9697e12dfbdcc..448bee3f3f1ce 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -440,6 +440,40 @@ int filemap_flush_range(struct address_space *mapping,= loff_t start, } EXPORT_SYMBOL_GPL(filemap_flush_range); =20 +/** + * filemap_dontcache_writeback_range - rate-limited writeback for dontcach= e I/O + * @mapping: target address_space + * @start: byte offset to start writeback + * @end: byte offset to end writeback (inclusive) + * @nr_written: number of bytes just written by the caller + * + * Kick writeback for dontcache I/O, but avoid piling on if writeback is + * already in progress. When writeback is kicked, limit the number of pag= es + * submitted to be proportional to the amount just written, rather than + * flushing the entire dirty range. + * + * This reduces tail latency compared to filemap_flush_range() which submi= ts + * writeback for all dirty pages on every call, creating queue contention + * under concurrent writers. + * + * Return: %0 on success, negative error code otherwise. + */ +int filemap_dontcache_writeback_range(struct address_space *mapping, + loff_t start, loff_t end, + ssize_t nr_written) +{ + long nr; + + /* If writeback is already active, don't pile on */ + if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) + return 0; + + nr =3D (nr_written + PAGE_SIZE - 1) >> PAGE_SHIFT; + return filemap_writeback(mapping, start, end, WB_SYNC_NONE, &nr, + WB_REASON_BACKGROUND); +} +EXPORT_SYMBOL_GPL(filemap_dontcache_writeback_range); + /** * filemap_flush - mostly a non-blocking flush * @mapping: target address_space @@ -2633,7 +2667,7 @@ static int filemap_create_folio(struct kiocb *iocb, s= truct folio_batch *fbatch) folio =3D filemap_alloc_folio(mapping_gfp_mask(mapping), min_order, NULL)= ; if (!folio) return -ENOMEM; - if (iocb->ki_flags & IOCB_DONTCACHE) + if (iocb->ki_flags & (IOCB_DONTCACHE | IOCB_DONTCACHE_LAZY)) __folio_set_dropbehind(folio); =20 /* @@ -2680,7 +2714,7 @@ static int filemap_readahead(struct kiocb *iocb, stru= ct file *file, =20 if (iocb->ki_flags & IOCB_NOIO) return -EAGAIN; - if (iocb->ki_flags & IOCB_DONTCACHE) + if (iocb->ki_flags & (IOCB_DONTCACHE | IOCB_DONTCACHE_LAZY)) ractl.dropbehind =3D 1; page_cache_async_ra(&ractl, folio, last_index - folio->index); return 0; @@ -2712,7 +2746,7 @@ static int filemap_get_pages(struct kiocb *iocb, size= _t count, return -EAGAIN; if (iocb->ki_flags & IOCB_NOWAIT) flags =3D memalloc_noio_save(); - if (iocb->ki_flags & IOCB_DONTCACHE) + if (iocb->ki_flags & (IOCB_DONTCACHE | IOCB_DONTCACHE_LAZY)) ractl.dropbehind =3D 1; page_cache_sync_ra(&ractl, last_index - index); if (iocb->ki_flags & IOCB_NOWAIT)