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

[Qiliang Yuan <odys.yuan@gmail.com>]

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>
---
 mm/filemap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index b74bc1e5015c6..a0c959e74533e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -140,7 +140,8 @@ 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 (shadow)
+		xas_init_marks(&xas);
 
 	folio->mapping = NULL;
 	/* Leave folio->index set: truncation lookup relies upon it */

---
base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3
change-id: 20261030-bug-mm-filemap-init-marks-once-b83c6aa67bab

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