[PATCH v5 0/2] mm: improve write performance with RWF_DONTCACHE

[PATCH v5 0/2] mm: improve write performance with RWF_DONTCACHE

[Jeff Layton]

This patch series is intended to improve write performce with
RWF_DONTCACHE. The main difference in this version is an updated comment
over filemap_dontcache_kick_writeback(), using the verbiage suggested
by Jan.

I've also dropped the two patches that add the benchmarks that I was
using for testing. I don't think they're appropriate for inclusion here,
though we could consider cleaning them up and adding them to xfstests or
something if there is interest.

Christian, please consider these for v7.2.

Thanks,

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Changes in v5:
- Flesh out comment over filemap_dontcache_kick_writeback()
- Drop testcases from posting
- Link to v4: https://lore.kernel.org/r/20260501-dontcache-v4-0-5d5e6dc71cb3@kernel.org

Changes in v4:
- Track DONTCACHE dirty pages per bdi_writeback
- New benchmark for competing buffered and dontcache writers
- New benchmark replicating Jens' original 32 concurrent writer test
- Link to v3: https://lore.kernel.org/r/20260426-dontcache-v3-0-79eb37da9547@kernel.org

Changes in v3:
- Track dirty DONTCACHE pages in the VM
- Have flusher write back a proportional number of pages after DONTCACHE write
- Link to v2: https://lore.kernel.org/r/20260408-dontcache-v2-0-948dec1e756b@kernel.org

Changes in v2:
- kick flusher thread instead of initiating writeback inline
- add mechanism to run 'perf lock' around the testcases
- Link to v1: https://lore.kernel.org/r/20260401-dontcache-v1-0-1f5746fab47a@kernel.org

---
Jeff Layton (2):
      mm: track DONTCACHE dirty pages per bdi_writeback
      mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking

 fs/fs-writeback.c                | 65 ++++++++++++++++++++++++++++++++++++++++
 include/linux/backing-dev-defs.h |  3 ++
 include/linux/fs.h               |  6 ++--
 include/trace/events/writeback.h |  3 +-
 mm/filemap.c                     | 13 +++++++-
 mm/page-writeback.c              |  6 ++++
 6 files changed, 90 insertions(+), 6 deletions(-)
---
base-commit: 7e2326f4275c11652e1fdaae11de06159fef1d90
change-id: 20260401-dontcache-5811efd7eaf3

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>

[PATCH v5 1/2] mm: track DONTCACHE dirty pages per bdi_writeback

[Jeff Layton]

Add a per-wb WB_DONTCACHE_DIRTY counter that tracks the number of dirty
pages with the dropbehind flag set (i.e., pages dirtied via RWF_DONTCACHE
writes).

Increment the counter alongside WB_RECLAIMABLE in folio_account_dirtied()
when the folio has the dropbehind flag set, and decrement it in
folio_clear_dirty_for_io() and folio_account_cleaned(). Also decrement it
when a non-DONTCACHE lookup clears the dropbehind flag on a dirty folio in
__filemap_get_folio_mpol(), using proper writeback domain locking.

The counter will be used by the writeback flusher to determine how many
pages to write back when expediting writeback for IOCB_DONTCACHE writes,
without flushing the entire BDI's dirty pages.

Suggested-by: Jan Kara <jack@suse.cz>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/backing-dev-defs.h |  1 +
 mm/filemap.c                     | 13 ++++++++++++-
 mm/page-writeback.c              |  6 ++++++
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index a06b93446d10..cb660dd37286 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -33,6 +33,7 @@ enum wb_stat_item {
 	WB_WRITEBACK,
 	WB_DIRTIED,
 	WB_WRITTEN,
+	WB_DONTCACHE_DIRTY,
 	NR_WB_STAT_ITEMS
 };
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 4e636647100c..1c9c0d5f495f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2052,8 +2052,19 @@ struct folio *__filemap_get_folio_mpol(struct address_space *mapping,
 	if (!folio)
 		return ERR_PTR(-ENOENT);
 	/* not an uncached lookup, clear uncached if set */
-	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE))
+	if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE)) {
+		if (folio_test_dirty(folio)) {
+			struct inode *inode = mapping->host;
+			struct bdi_writeback *wb;
+			struct wb_lock_cookie cookie = {};
+
+			wb = unlocked_inode_to_wb_begin(inode, &cookie);
+			wb_stat_mod(wb, WB_DONTCACHE_DIRTY,
+				    -folio_nr_pages(folio));
+			unlocked_inode_to_wb_end(inode, &cookie);
+		}
 		folio_clear_dropbehind(folio);
+	}
 	return folio;
 }
 EXPORT_SYMBOL(__filemap_get_folio_mpol);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 88cd53d4ba09..8e520717d1f6 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2630,6 +2630,8 @@ static void folio_account_dirtied(struct folio *folio,
 		wb = inode_to_wb(inode);
 
 		lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, nr);
+		if (folio_test_dropbehind(folio))
+			wb_stat_mod(wb, WB_DONTCACHE_DIRTY, nr);
 		__zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, nr);
 		__node_stat_mod_folio(folio, NR_DIRTIED, nr);
 		wb_stat_mod(wb, WB_RECLAIMABLE, nr);
@@ -2651,6 +2653,8 @@ void folio_account_cleaned(struct folio *folio, struct bdi_writeback *wb)
 	long nr = folio_nr_pages(folio);
 
 	lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
+	if (folio_test_dropbehind(folio))
+		wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
 	zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
 	wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
 	task_io_account_cancelled_write(nr * PAGE_SIZE);
@@ -2920,6 +2924,8 @@ bool folio_clear_dirty_for_io(struct folio *folio)
 		if (folio_test_clear_dirty(folio)) {
 			long nr = folio_nr_pages(folio);
 			lruvec_stat_mod_folio(folio, NR_FILE_DIRTY, -nr);
+			if (folio_test_dropbehind(folio))
+				wb_stat_mod(wb, WB_DONTCACHE_DIRTY, -nr);
 			zone_stat_mod_folio(folio, NR_ZONE_WRITE_PENDING, -nr);
 			wb_stat_mod(wb, WB_RECLAIMABLE, -nr);
 			ret = true;

-- 
2.54.0

[PATCH v5 2/2] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking

[Jeff Layton]

The IOCB_DONTCACHE writeback path in generic_write_sync() calls
filemap_flush_range() on every write, submitting writeback inline in
the writer's context.  Perf lock contention profiling shows the
performance problem is not lock contention but the writeback submission
work itself — walking the page tree and submitting I/O blocks the writer
for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms
(dontcache).

Replace the inline filemap_flush_range() call with a flusher kick that
drains dirty pages in the background.  This moves writeback submission
completely off the writer's hot path.

To avoid flushing unrelated buffered dirty data, add a dedicated
WB_start_dontcache bit and wb_check_start_dontcache() handler that uses
the per-wb WB_DONTCACHE_DIRTY counter to determine how many pages to
write back.  The flusher writes back that many pages from the oldest dirty
inodes (not restricted to dontcache-specific inodes). This helps
preserve I/O batching while limiting the scope of expedited writeback.

Like WB_start_all, the WB_start_dontcache bit coalesces multiple
DONTCACHE writes into a single flusher wakeup without per-write
allocations.

Also add WB_REASON_DONTCACHE as a new writeback reason for tracing
visibility, and target the correct cgroup writeback domain via
unlocked_inode_to_wb_begin().

dontcache-bench results (same host, T6F_SKL_1920GBF, 251 GiB RAM,
xfs on NVMe, fio io_uring):

Buffered and direct I/O paths are unaffected by this patchset. All
improvements are confined to the dontcache path:

Single-stream throughput (MB/s):
                        Before    After    Change
  seq-write/dontcache      298      897    +201%
  rand-write/dontcache     131      236     +80%

Tail latency improvements (seq-write/dontcache):
  p99:    135,266 us  ->  23,986 us   (-82%)
  p99.9: 8,925,479 us ->  28,443 us   (-99.7%)

Multi-writer (4 jobs, sequential write):
                                Before    After    Change
  dontcache aggregate (MB/s)     2,529    4,532     +79%
  dontcache p99 (us)             8,553    1,002     -88%
  dontcache p99.9 (us)         109,314    1,057     -99%

  Dontcache multi-writer throughput now matches buffered (4,532 vs
  4,616 MB/s).

32-file write (Axboe test):
                                Before    After    Change
  dontcache aggregate (MB/s)     1,548    3,499    +126%
  dontcache p99 (us)            10,170      602     -94%
  Peak dirty pages (MB)          1,837      213     -88%

  Dontcache now reaches 81% of buffered throughput (was 35%).

Competing writers (dontcache vs buffered, separate files):
                                Before    After
  buffered writer                  868      433 MB/s
  dontcache writer                 415      433 MB/s
  Aggregate                      1,284      866 MB/s

  Previously the buffered writer starved the dontcache writer 2:1.
  With per-bdi_writeback tracking, both writers now receive equal
  bandwidth. The aggregate matches the buffered-vs-buffered baseline
  (863 MB/s), indicating fair sharing regardless of I/O mode.

  The dontcache writer's p99.9 latency collapsed from 119 ms to
  33 ms (-73%), eliminating the severe periodic stalls seen in the
  baseline. Both writers now share identical latency profiles,
  matching the buffered-vs-buffered pattern.

The per-bdi_writeback dirty tracking dramatically reduces peak dirty
pages in dontcache workloads, with the 32-file test dropping from
1.8 GB to 213 MB. Dontcache sequential write throughput triples and
multi-writer throughput reaches parity with buffered I/O, with tail
latencies collapsing by 1-2 orders of magnitude.

Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/fs-writeback.c                | 65 ++++++++++++++++++++++++++++++++++++++++
 include/linux/backing-dev-defs.h |  2 ++
 include/linux/fs.h               |  6 ++--
 include/trace/events/writeback.h |  3 +-
 4 files changed, 71 insertions(+), 5 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a65694cbfe68..ebef485b2f8b 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1334,6 +1334,18 @@ static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)
 	wb_wakeup(wb);
 }
 
+static void wb_start_dontcache_writeback(struct bdi_writeback *wb)
+{
+	if (!wb_has_dirty_io(wb))
+		return;
+
+	if (test_bit(WB_start_dontcache, &wb->state) ||
+	    test_and_set_bit(WB_start_dontcache, &wb->state))
+		return;
+
+	wb_wakeup(wb);
+}
+
 /**
  * wb_start_background_writeback - start background writeback
  * @wb: bdi_writback to write from
@@ -2373,6 +2385,28 @@ static long wb_check_start_all(struct bdi_writeback *wb)
 	return nr_pages;
 }
 
+static long wb_check_start_dontcache(struct bdi_writeback *wb)
+{
+	long nr_pages;
+
+	if (!test_bit(WB_start_dontcache, &wb->state))
+		return 0;
+
+	nr_pages = wb_stat(wb, WB_DONTCACHE_DIRTY);
+	if (nr_pages) {
+		struct wb_writeback_work work = {
+			.nr_pages	= nr_pages,
+			.sync_mode	= WB_SYNC_NONE,
+			.range_cyclic	= 1,
+			.reason		= WB_REASON_DONTCACHE,
+		};
+
+		nr_pages = wb_writeback(wb, &work);
+	}
+
+	clear_bit(WB_start_dontcache, &wb->state);
+	return nr_pages;
+}
 
 /*
  * Retrieve work items and do the writeback they describe
@@ -2394,6 +2428,11 @@ static long wb_do_writeback(struct bdi_writeback *wb)
 	 */
 	wrote += wb_check_start_all(wb);
 
+	/*
+	 * Check for dontcache writeback request
+	 */
+	wrote += wb_check_start_dontcache(wb);
+
 	/*
 	 * Check for periodic writeback, kupdated() style
 	 */
@@ -2468,6 +2507,32 @@ void wakeup_flusher_threads_bdi(struct backing_dev_info *bdi,
 	rcu_read_unlock();
 }
 
+/**
+ * filemap_dontcache_kick_writeback - kick flusher for IOCB_DONTCACHE writes
+ * @mapping:	address_space that was just written to
+ *
+ * Kick the writeback flusher thread to expedite writeback of dontcache dirty
+ * pages. Queue writeback for the inode's wb for as many pages as there are
+ * dontcache pages, but don't restrict writeback to dontcache pages only.
+ *
+ * This significantly improves performance over either writing all wb's pages
+ * or writing only dontcache pages.  Although it doesn't guarantee quick
+ * writeback and reclaim of dontcache pages, it keeps the amount of dirty pages
+ * in check. Over longer term dontcache pages get written and reclaimed by
+ * background writeback even with this rough heuristic.
+ */
+void filemap_dontcache_kick_writeback(struct address_space *mapping)
+{
+	struct inode *inode = mapping->host;
+	struct bdi_writeback *wb;
+	struct wb_lock_cookie cookie = {};
+
+	wb = unlocked_inode_to_wb_begin(inode, &cookie);
+	wb_start_dontcache_writeback(wb);
+	unlocked_inode_to_wb_end(inode, &cookie);
+}
+EXPORT_SYMBOL_GPL(filemap_dontcache_kick_writeback);
+
 /*
  * Wakeup the flusher threads to start writeback of all currently dirty pages
  */
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index cb660dd37286..4f1084937315 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -26,6 +26,7 @@ enum wb_state {
 	WB_writeback_running,	/* Writeback is in progress */
 	WB_has_dirty_io,	/* Dirty inodes on ->b_{dirty|io|more_io} */
 	WB_start_all,		/* nr_pages == 0 (all) work pending */
+	WB_start_dontcache,	/* dontcache writeback pending */
 };
 
 enum wb_stat_item {
@@ -56,6 +57,7 @@ enum wb_reason {
 	 */
 	WB_REASON_FORKER_THREAD,
 	WB_REASON_FOREIGN_FLUSH,
+	WB_REASON_DONTCACHE,
 
 	WB_REASON_MAX,
 };
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 11559c513dfb..df72b42a9e9b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2624,6 +2624,7 @@ extern int __must_check file_write_and_wait_range(struct file *file,
 						loff_t start, loff_t end);
 int filemap_flush_range(struct address_space *mapping, loff_t start,
 		loff_t end);
+void filemap_dontcache_kick_writeback(struct address_space *mapping);
 
 static inline int file_write_and_wait(struct file *file)
 {
@@ -2657,10 +2658,7 @@ static inline ssize_t generic_write_sync(struct kiocb *iocb, ssize_t count)
 		if (ret)
 			return ret;
 	} else if (iocb->ki_flags & IOCB_DONTCACHE) {
-		struct address_space *mapping = iocb->ki_filp->f_mapping;
-
-		filemap_flush_range(mapping, iocb->ki_pos - count,
-				iocb->ki_pos - 1);
+		filemap_dontcache_kick_writeback(iocb->ki_filp->f_mapping);
 	}
 
 	return count;
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index bdac0d685a98..13ee076ccd16 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -44,7 +44,8 @@
 	EM( WB_REASON_PERIODIC,			"periodic")		\
 	EM( WB_REASON_FS_FREE_SPACE,		"fs_free_space")	\
 	EM( WB_REASON_FORKER_THREAD,		"forker_thread")	\
-	EMe(WB_REASON_FOREIGN_FLUSH,		"foreign_flush")
+	EM( WB_REASON_FOREIGN_FLUSH,		"foreign_flush")	\
+	EMe(WB_REASON_DONTCACHE,		"dontcache")
 
 WB_WORK_REASON
 

-- 
2.54.0