[PATCH v2] mm/filemap: clear page cache marks once when removing a folio

[PATCH v2] mm/filemap: clear page cache marks once when removing a folio

[Qiliang Yuan]

xas_store() clears the marks of an entry before it replaces the entry
with NULL.

page_cache_delete() calls xas_init_marks() after xas_store() whether it
stored a shadow entry or NULL, so a folio removed without a shadow, as
POSIX_FADV_DONTNEED and drop_caches do, has its marks cleared twice.
With 4K folios that is about 6% of the time it takes to drop clean page
cache.

Call xas_init_marks() only after storing a shadow entry, whose marks
xas_store() leaves alone. Dropping 8 GiB of 4K folios, median of five
runs, averaged over two boots of each kernel, in a 32 vCPU VM:

                          before    after
  POSIX_FADV_DONTNEED    1517 ms  1423 ms
  drop_caches            1515 ms  1422 ms

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
V1 -> V2:
- Explain the condition in a comment (Jan Kara)
- Add Reviewed-by from Jan
- Rebase on v7.4-rc1

v1: https://lore.kernel.org/r/20261030-bug-mm-filemap-init-marks-once-v1-1-699835fc6690@gmail.com
---
 mm/filemap.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index b74bc1e5015c6..c2effedf975a2 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -140,7 +140,9 @@ static void page_cache_delete(struct address_space *mapping,
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
 	xas_store(&xas, shadow);
-	xas_init_marks(&xas);
+	/* If xas_store() didn't clear marks, do it now */
+	if (shadow)
+		xas_init_marks(&xas);
 
 	folio->mapping = NULL;
 	/* Leave folio->index set: truncation lookup relies upon it */

---
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
change-id: 20261030-bug-mm-filemap-init-marks-once-b83c6aa67bab

Best regards,
-- 
Qiliang Yuan <odys.yuan@gmail.com>