[PATCH] mm: filemap: move lruvec accounting outside the xarray lock

[Usama Arif <usama.arif@linux.dev>]

__filemap_add_folio() inserts a folio and updates mapping->nrpages
while holding mapping->i_pages.xa_lock with interrupts disabled. The
XArray insertion and nrpages update require the lock, but the lruvec
statistic updates do not. With CONFIG_MEMCG, those calls also update
per-CPU memcg and lruvec counters and notify cgroup rstat, extending the
critical section.

Move the lruvec accounting after a successful XArray insertion and
after xas_unlock_irq(). The page-cache references pin the folio, while
the folio lock keeps folio->mapping stable and prevents removal until
accounting is complete. This moves one lruvec update for ordinary folios
and a second for PMD-mappable folios out of the serialized section.

In a 30-second system-wide perf lock contention -ab capture on a
production host, the hottest caller-stack record attributed to
__filemap_add_folio() had 20,867 contentions and 557.930 ms total wait.
That was 14% of the 3.998 seconds of aggregate lock wait in the
capture. Moving lruvec accuting outside of critical section should
help optimize it.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/filemap.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 00fd89cf6f550..4720bbfc1a663 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -918,14 +918,6 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 
 		mapping->nrpages += nr;
 
-		/* hugetlb pages do not participate in page cache accounting */
-		if (!huge) {
-			lruvec_stat_mod_folio(folio, NR_FILE_PAGES, nr);
-			if (folio_test_pmd_mappable(folio))
-				lruvec_stat_mod_folio(folio,
-						NR_FILE_THPS, nr);
-		}
-
 unlock:
 		xas_unlock_irq(&xas);
 
@@ -942,6 +934,13 @@ noinline int __filemap_add_folio(struct address_space *mapping,
 	if (xas_error(&xas))
 		goto error;
 
+	/* hugetlb pages do not participate in page cache accounting */
+	if (!huge) {
+		lruvec_stat_mod_folio(folio, NR_FILE_PAGES, nr);
+		if (folio_test_pmd_mappable(folio))
+			lruvec_stat_mod_folio(folio, NR_FILE_THPS, nr);
+	}
+
 	trace_mm_filemap_add_to_page_cache(folio);
 	return 0;
 error:
-- 
2.53.0-Meta