[PATCH RFC v3 14/17] mm/mglru: reparent folios from all generations

[Kairui Song via B4 Relay]

From: Kairui Song <kasong@xxxxxxxxxxx>

A lockless promotion racing with aging can reuse a generation index,
see folio_inc_lru_refs().  The retry repairs the folio flags, but
sort_folio() may have already moved the folio to the list of a
generation outside the [min_seq, max_seq] window.

__lru_gen_reparent_memcg() only splices the lists inside the child's
window, so such a folio would be left on a list of the dying memcg.
Splice the lists and move the counters of all MAX_NR_GENS generations
instead.  Those outside the window are empty in the common case.

Assisted-by: LLM
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/vmscan.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9cc06a95d931..1bc8e5056265 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4947,7 +4947,8 @@ static void __lru_gen_reparent_memcg(struct lruvec *child_lruvec, struct lruvec
 	child_lrugen = &child_lruvec->lrugen;
 	parent_lrugen = &parent_lruvec->lrugen;
 
-	for (i = 0; i < get_nr_gens(child_lruvec, type); i++) {
+	/* Lockless promotion may leave folios outside the child's window */
+	for (i = 0; i < MAX_NR_GENS; i++) {
 		int gen = lru_gen_from_seq(child_lrugen->max_seq - i);
 		long nr_pages = atomic_long_read(&child_lrugen->nr_pages[gen][type][zone]);
 

-- 
2.55.0