[PATCH v2] mm: do not update memcg stats for NR_{FILE/SHMEM}_PMDMAPPED

[Yosry Ahmed]

Previously, all NR_VM_EVENT_ITEMS stats were maintained per-memcg,
although some of those fields are not exposed anywhere. Commit
14e0f6c957e39 ("memcg: reduce memory for the lruvec and memcg stats")
changed this such that we only maintain the stats we actually expose
per-memcg via a translation table.

Additionally, commit 514462bbe927b ("memcg: warn for unexpected events
and stats") added a warning if a per-memcg stat update is attempted for
a stat that is not in the translation table. The warning started firing
for the NR_{FILE/SHMEM}_PMDMAPPED stat updates in the rmap code. These
stats are not maintained per-memcg, and hence are not in the translation
table.

Do not use __lruvec_stat_mod_folio() when updating NR_FILE_PMDMAPPED and
NR_SHMEM_PMDMAPPED. Use __mod_node_page_state() instead, which updates
the global per-node stats only.

Reported-by: syzbot+9319a4268a640e26b72b@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/lkml/0000000000001b9d500617c8b23c@xxxxxxxxxx
Fixes: 514462bbe927 ("memcg: warn for unexpected events and stats")
Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
 mm/rmap.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 12be4241474ab..ed7f820369864 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1435,13 +1435,14 @@ static __always_inline void __folio_add_file_rmap(struct folio *folio,
 		struct page *page, int nr_pages, struct vm_area_struct *vma,
 		enum rmap_level level)
 {
+	pg_data_t *pgdat = folio_pgdat(folio);
 	int nr, nr_pmdmapped = 0;
 
 	VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
 
 	nr = __folio_add_rmap(folio, page, nr_pages, level, &nr_pmdmapped);
 	if (nr_pmdmapped)
-		__lruvec_stat_mod_folio(folio, folio_test_swapbacked(folio) ?
+		__mod_node_page_state(pgdat, folio_test_swapbacked(folio) ?
 			NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED, nr_pmdmapped);
 	if (nr)
 		__lruvec_stat_mod_folio(folio, NR_FILE_MAPPED, nr);
@@ -1493,6 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 		enum rmap_level level)
 {
 	atomic_t *mapped = &folio->_nr_pages_mapped;
+	pg_data_t *pgdat = folio_pgdat(folio);
 	int last, nr = 0, nr_pmdmapped = 0;
 	bool partially_mapped = false;
 	enum node_stat_item idx;
@@ -1540,13 +1542,14 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 	}
 
 	if (nr_pmdmapped) {
+		/* NR_{FILE/SHMEM}_PMDMAPPED are not maintained per-memcg */
 		if (folio_test_anon(folio))
-			idx = NR_ANON_THPS;
-		else if (folio_test_swapbacked(folio))
-			idx = NR_SHMEM_PMDMAPPED;
+			__lruvec_stat_mod_folio(folio, NR_ANON_THPS, -nr_pmdmapped);
 		else
-			idx = NR_FILE_PMDMAPPED;
-		__lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped);
+			__mod_node_page_state(pgdat,
+					folio_test_swapbacked(folio) ?
+					NR_SHMEM_PMDMAPPED : NR_FILE_PMDMAPPED,
+					-nr_pmdmapped);
 	}
 	if (nr) {
 		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
-- 
2.45.0.rc1.225.g2a3ae87e7f-goog