[RFC PATCH v5 04/14] mm/mglru: lift the forced protection of reclaimer

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

From: Kairui Song <kasong@tencent.com>

When a type is drained down to MIN_NR_GENS gens, the reclaimer will
fail to make any progress on that type without aging.

However aging comes with a cost, and it will take time to finish. Even
if we do aging more proactively, when under great pressure, gens will
still be drained very fast so the new gens produced by aging can't catch
up with the consumption. With MIN_NR_GENS setting blocking the reclaim,
we will run into OOM. This OOM can be reproduced by the previous test
program [1] used for fixing another aging problem [2]. When running in
great pressure, certain workloads will keep seeing premature OOM with
MGLRU, and never see any OOM with classical LRU.

This might get worse if we implement async aging, or avoid hotness
inversion in aging, as they will slow down the aging process even more.

To fix this, let's first drop the get_nr_gens() == MIN_NR_GENS early
return and let the raised-priority scan bound cover max_seq itself.

Mapped folios will still survive as the rmap PTE check will keep them.
Unmapped cache folios may thrashing more when under great pressure,
which is really a problem that should be fixed with refault distance
and frequecy aware promotoin instead [3].

Link: https://github.com/ryncsn/emm-test-project/tree/master/file-anon-mix-pressure [1]
Link: https://lore.kernel.org/linux-mm/20260428-mglru-reclaim-v7-0-02fabb92dc43@tencent.com/ [2]
Link: https://lore.kernel.org/linux-mm/20260502-mglru-fg-v1-0-913619b014d9@tencent.com/ [3]
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d7eb15533de1..2cccaa5dd4e7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4834,7 +4834,7 @@ void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent,
  ******************************************************************************/
 
 static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_control *sc,
-		       int min_gen, int tier_idx)
+		       int min_gen, int max_gen, int tier_idx)
 {
 	int gen = folio_lru_gen(folio);
 	int type = folio_is_file_lru(folio);
@@ -4851,12 +4851,22 @@ static bool sort_folio(struct lruvec *lruvec, struct folio *folio, struct scan_c
 	if (!folio_evictable(folio))
 		return false;
 
-	/* promoted */
+	/* promoted, or moved due to reparenting. */
 	if (gen != min_gen) {
 		list_move(&folio->lru, &lrugen->folios[gen][type][zone]);
 		return true;
 	}
 
+	/*
+	 * We are reclaiming the last gen left. Folios in this gen can no
+	 * longer be promoted or protected, so just isolate them. The one
+	 * exception is lru_gen_reparent_memcg(), which is handled above.
+	 */
+	if (min_gen == max_gen) {
+		VM_WARN_ON_ONCE(zone > sc->reclaim_idx);
+		return false;
+	}
+
 	/* protected */
 	if (tier > tier_idx || refs + workingset == BIT(LRU_REFS_WIDTH) + 1) {
 		gen = folio_inc_gen(lruvec, folio, min_gen);
@@ -4911,15 +4921,16 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 		       struct list_head *list, int *isolatedp)
 {
 	int i;
-	int gen;
 	enum node_stat_item item;
 	int sorted = 0;
 	int scanned = 0;
 	int isolated = 0;
 	int skipped = 0;
+	int gen, max_gen;
 	unsigned long min_seq;
 	unsigned long remaining = nr_to_scan;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	DEFINE_MAX_SEQ(lruvec);
 
 	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
 	VM_WARN_ON_ONCE(!list_empty(list));
@@ -4932,18 +4943,24 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	if (sc->priority == DEF_PRIORITY)
 		min_seq = lrugen->min_seq[type];
 	else
-		min_seq = lruvec_populated_min_seq(lruvec, type, lrugen->max_seq - MIN_NR_GENS + 1);
-
-	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
-		return 0;
+		min_seq = lruvec_populated_min_seq(lruvec, type, lrugen->max_seq);
 
 	gen = lru_gen_from_seq(min_seq);
+	max_gen = lru_gen_from_seq(max_seq);
+
 	for (i = MAX_NR_ZONES; i > 0; i--) {
 		LIST_HEAD(moved);
 		int skipped_zone = 0;
 		int zone = (sc->reclaim_idx + i) % MAX_NR_ZONES;
 		struct list_head *head = &lrugen->folios[gen][type][zone];
 
+		/*
+		 * If there is only one gen left, rotating the unreclaimable
+		 * zone become meaningless, just skip them.
+		 */
+		if (gen == max_gen && zone > sc->reclaim_idx)
+			continue;
+
 		while (!list_empty(head)) {
 			struct folio *folio = lru_to_folio(head);
 			int delta = folio_nr_pages(folio);
@@ -4954,8 +4971,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
 
 			scanned += delta;
-
-			if (sort_folio(lruvec, folio, sc, gen, tier))
+			if (sort_folio(lruvec, folio, sc, gen, max_gen, tier))
 				sorted += delta;
 			else if (isolate_folio(lruvec, folio, sc)) {
 				list_add(&folio->lru, list);
-- 
2.34.1