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[68.160.167.46]) by smtp.gmail.com with ESMTPSA id af79cd13be357-93f56d98978sm89964885a.10.2026.10.20.15.03.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 15:03:55 -0700 (PDT) Sender: Mike Snitzer From: Mike Snitzer X-Google-Original-From: Mike Snitzer To: Chuck Lever , Jeff Layton 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, Jonathan Flynn Subject: [PATCH v7 5/5] NFSD: drop a shared boundary page only once both WRITEs have written it Date: Tue, 20 Oct 2026 18:03:46 -0400 Message-ID: <20261020220346.28020-6-snitzer@kernel.org> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20261020220346.28020-1-snitzer@kernel.org> References: <20261020220346.28020-1-snitzer@kernel.org> X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit A buffered DONTCACHE segment of a direct-mode WRITE -- the misaligned start or end of a split WRITE, or the whole of one too small to split -- covers its first and last page only partly when it does not start or end on a page boundary. Such a page is shared with the neighbouring WRITE, which may arrive before, after or at the same time, from any client. The page cache keeps a page an uncached write covers only partly for the write that completes it ("mm, iomap: keep a partly covered page until an uncached write completes it"), so the second WRITE does not have to read it back from disk. Left at that, the page cache marks the page drop- behind while it is still dirty, which counts it as DONTCACHE dirty and wakes the DONTCACHE writeback kick for every segment of a stable WRITE, only to write back what NFSD's single fsync is about to write. Issue the buffered segments IOCB_DONTCACHE_DEFER and mark the pages here instead ("mm, iomap: let an uncached writer complete its shared pages itself"). Immediately before writing a segment, NFSD asks filemap_dontcache_edge_completes() whether each page it covers only partly has already been written by the neighbouring WRITE; once the whole WRITE, its fsync included, is complete, it calls filemap_dontcache_edge_complete() for those pages, which drops a clean page at once. An UNSTABLE WRITE marks them dirty, and the writeback that cleans them, by the flusher or for the client's COMMIT, drops them. A page whose neighbouring WRITE never arrives, such as the last page of the file, or arrives at the same time, so that neither finds the other's part, stays cached until it is reclaimed. Nothing changes when the buffered segments are normal buffered I/O (direct_misaligned_dontcache is N, or the file system lacks FOP_DONTCACHE); the page cache keeps partly covered pages only for file systems that use iomap for buffered writes. Measured with 32 interleaved writers of 47008-byte records over an emulated 4Kn NVMe with 4 KiB pages, NFSD DIRECT with direct_misaligned_dontcache=Y, each run from a freshly made file system: device reads per WRITE are 0.0001-0.0002 whether each record is one FILE_SYNC WRITE, two sent one after the other, two sent together, or two sent UNSTABLE, against 45144 reads for 45664 WRITEs with no shared-page handling, and the DONTCACHE writeback kick does not run at all for the FILE_SYNC WRITEs. On a four-server pNFS flexfiles rig (x86_64, 4 KiB logical blocks), three clients writing 47008-byte records for 240 s against 640 nfsd threads per server, keeping the shared page took device reads from 0.014-0.015 per record to 0.003, page-cache growth over the write phase from about 11 GiB to 2.9 GiB, and write throughput up by 2.4% to 4.9%, with read throughput unchanged. Keeping every boundary page cached instead, with direct_misaligned_dontcache=N, grows the page cache by about 630 GiB over the same runs. Reported-by: Jonathan Flynn Assisted-by: LLM Signed-off-by: Mike Snitzer --- .../filesystems/nfs/nfsd-io-modes.rst | 17 ++++ fs/nfsd/vfs.c | 99 ++++++++++++++++--- 2 files changed, 104 insertions(+), 12 deletions(-) diff --git a/Documentation/filesystems/nfs/nfsd-io-modes.rst b/Documentation/filesystems/nfs/nfsd-io-modes.rst index 0e69b5fdd675..9c663ce97c48 100644 --- a/Documentation/filesystems/nfs/nfsd-io-modes.rst +++ b/Documentation/filesystems/nfs/nfsd-io-modes.rst @@ -167,6 +167,23 @@ Misaligned WRITE: above. Set it to N for a workload that reads back or rewrites what it just wrote. + The page holding a start or end segment is shared with the + neighbouring WRITE, which may arrive before, after or at the same + time, from any client. So that the second WRITE to such a page does + not have to read it back from disk, the page cache keeps a page an + uncached write covers only partly, unmarked, for the write that + completes it. NFSD asks, immediately before writing a start or end + segment, whether the neighbouring WRITE has already written its + part; if it has, NFSD marks the page DONTCACHE once its own WRITE, + including its fsync, is complete, and the page is dropped. A page + whose neighbouring WRITE never arrives, or arrives at the same time, + stays cached until it is reclaimed. The first and last pages of a + WRITE that is not split are handled the same way. + + None of this applies when the start and end segments are normal + buffered IO, or for a filesystem that does not use iomap for + buffered writes. + Tracing: The nfsd_read_direct trace event marks a READ that NFSD issues as O_DIRECT. It records the offset and length the client requested. diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c index 9219f8eb386d..3605ddf1ad1e 100644 --- a/fs/nfsd/vfs.c +++ b/fs/nfsd/vfs.c @@ -1387,6 +1387,7 @@ static int wait_for_concurrent_writes(struct file *file) struct nfsd_write_dio_seg { struct iov_iter iter; int flags; + bool claim; /* completes shared pages */ }; static unsigned long @@ -1406,6 +1407,65 @@ nfsd_write_dio_seg_init(struct nfsd_write_dio_seg *segment, iov_iter_advance(&segment->iter, start); iov_iter_truncate(&segment->iter, len); segment->flags = iocb->ki_flags; + segment->claim = false; +} + +/* The shared pages a segment completes, and how much of it was written. */ +struct nfsd_write_dio_hold { + loff_t first, last; + bool complete_first, complete_last; + size_t written; +}; + +/* + * A page that a buffered DONTCACHE segment covers only partly is shared with + * the neighbouring WRITE. The page cache keeps it, unmarked, for the write + * that completes it; the segment is issued IOCB_DONTCACHE_DEFER so that this + * WRITE, not the page cache, marks it, once the whole WRITE, its fsync + * included, is complete. Ask whether the neighbour has already written its + * part immediately before writing @seg: asking earlier gives the neighbour + * time to write its part and find this WRITE's part missing. + */ +static void +nfsd_write_dio_seg_hold(struct file *file, const struct nfsd_write_dio_seg *seg, + loff_t pos, size_t len, struct nfsd_write_dio_hold *hold) +{ + struct address_space *mapping = file->f_mapping; + + hold->first = pos; + hold->last = pos + len - 1; + hold->complete_first = false; + hold->complete_last = false; + hold->written = 0; + if (seg->claim && len) { + bool one_page = (hold->first >> PAGE_SHIFT) == + (hold->last >> PAGE_SHIFT); + bool head = hold->first & ~PAGE_MASK; + bool tail = (hold->last + 1) & ~PAGE_MASK; + + /* Only a page the segment covers partly is shared. */ + if (head || (one_page && tail)) + hold->complete_first = + filemap_dontcache_edge_completes(mapping, hold->first); + if (!one_page && tail) + hold->complete_last = + filemap_dontcache_edge_completes(mapping, hold->last); + } +} + +/* Drop the shared pages this WRITE was the second of two to write. */ +static void +nfsd_write_dio_seg_release(struct file *file, + const struct nfsd_write_dio_hold *hold) +{ + struct address_space *mapping = file->f_mapping; + + if (hold->complete_first && hold->written) + filemap_dontcache_edge_complete(mapping, hold->first); + /* A short write did not reach the last page. */ + if (hold->complete_last && + hold->written == hold->last - hold->first + 1) + filemap_dontcache_edge_complete(mapping, hold->last); } static unsigned int @@ -1428,7 +1488,9 @@ nfsd_write_dio_iters_init(struct svc_rqst *rqstp, struct svc_fh *fhp, if (nf->nf_file->f_op->fop_flags & FOP_DONTCACHE) dontcache_flags = IOCB_DONTCACHE; - buffered_flags = nfsd_direct_misaligned_dontcache ? dontcache_flags : 0; + /* NFSD marks their shared pages itself, see nfsd_write_dio_seg_hold() */ + buffered_flags = nfsd_direct_misaligned_dontcache && dontcache_flags ? + dontcache_flags | IOCB_DONTCACHE_DEFER : 0; /* * Check if direct I/O is feasible for this write request. @@ -1483,7 +1545,8 @@ nfsd_write_dio_iters_init(struct svc_rqst *rqstp, struct svc_fh *fhp, if (prefix) { nfsd_write_dio_seg_init(&segments[nsegs], bvec, nvecs, total, 0, prefix, iocb); - segments[nsegs++].flags |= buffered_flags; + segments[nsegs].flags |= buffered_flags; + segments[nsegs++].claim = !!buffered_flags; } nfsd_write_dio_seg_init(&segments[nsegs], bvec, nvecs, @@ -1516,7 +1579,8 @@ nfsd_write_dio_iters_init(struct svc_rqst *rqstp, struct svc_fh *fhp, if (suffix) { nfsd_write_dio_seg_init(&segments[nsegs], bvec, nvecs, total, prefix + middle, suffix, iocb); - segments[nsegs++].flags |= buffered_flags; + segments[nsegs].flags |= buffered_flags; + segments[nsegs++].claim = !!buffered_flags; } return nsegs; @@ -1525,6 +1589,7 @@ nfsd_write_dio_iters_init(struct svc_rqst *rqstp, struct svc_fh *fhp, nfsd_write_dio_seg_init(&segments[0], bvec, nvecs, total, 0, total, iocb); segments[0].flags |= buffered_flags; + segments[0].claim = !!buffered_flags; return 1; } @@ -1533,12 +1598,13 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf, unsigned int nvecs, unsigned long *cnt, struct kiocb *kiocb) { + struct nfsd_write_dio_hold holds[3]; struct nfsd_write_dio_seg segments[3]; struct file *file = nf->nf_file; loff_t start = kiocb->ki_pos; - unsigned int nsegs, i; + unsigned int nsegs, nheld = 0, i; + ssize_t host_err = 0; int sync_flags; - ssize_t host_err; size_t expected; nsegs = nfsd_write_dio_iters_init(rqstp, fhp, nf, rqstp->rq_bvec, @@ -1567,23 +1633,32 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp, kiocb->ki_flags); expected = iov_iter_count(&segments[i].iter); - + nfsd_write_dio_seg_hold(file, &segments[i], kiocb->ki_pos, + expected, &holds[nheld]); host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); if (host_err < 0) - return host_err; + break; + holds[nheld++].written = host_err; *cnt += host_err; if (host_err < (ssize_t)expected) break; /* partial write */ } - if (sync_flags && *cnt) { + if (host_err >= 0 && sync_flags && *cnt) host_err = vfs_fsync_range(file, start, start + *cnt - 1, !(sync_flags & IOCB_SYNC)); - if (host_err < 0) - return host_err; - } - return 0; + /* + * Mark the pages this WRITE shares with its neighbours for dropping + * only once the WRITE is complete, including its fsync if it has one. + * A page marked while still dirty is counted as DONTCACHE dirty and + * wakes the DONTCACHE writeback kick; a page marked once clean is + * simply dropped. + */ + for (i = 0; i < nheld; i++) + nfsd_write_dio_seg_release(file, &holds[i]); + + return host_err < 0 ? host_err : 0; } /** -- 2.52.0