[PATCH 3/6] mm/mglru: enhance cold/hot inversion handling in inc_min_seq()

["Barry Song (Xiaomi)" <baohua@kernel.org>]

During aging, a folio's generation may already have been updated by
folio_update_gen(), even though it has not yet been moved to the
corresponding generation list. Such folios are hotter than those
already in that generation.

It makes sense for inc_min_seq() to increment the generation of
folios that were never promoted during aging and move them to the
tail of the new oldest generation. However, folios that were already
promoted should instead be moved to the head of their updated
generation, just as sort_folio() does in scan_folios().

Otherwise, promoted folios could end up behind folios that were
never promoted, effectively inverting their hot/cold ordering.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 99ee3c833d54..3b618a51cde2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3946,9 +3946,12 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
 			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
 
 			new_gen = __folio_inc_gen(folio, old_gen, &gen_increased);
-			list_move_tail(&folio->lru, &lrugen->folios[new_gen][type][zone]);
-			if (gen_increased)
+			if (gen_increased) {
 				delta += nr_pages;
+				list_move_tail(&folio->lru, &lrugen->folios[new_gen][type][zone]);
+			} else {
+				list_move(&folio->lru, &lrugen->folios[new_gen][type][zone]);
+			}
 			/* don't count the workingset being lazily promoted */
 			if (refs + workingset != BIT(LRU_REFS_WIDTH) + 1) {
 				int tier = lru_tier_from_refs(refs, workingset);
-- 
2.34.1