[RFC PATCH v3 2/6] mm: mglru: let scan_folios() scan both reclaimable generations

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

When we have four generations, and scan_folios() exhausts the oldest
one while the second-oldest still contains reclaimable folios, the
current implementation doesn't move on to scan the second-oldest
generation. Instead, it returns, leaving that generation with no
chance to be scanned at the current sc->priority.

This doesn't seem right. Rather than breaking out and starting a
larger next iteration, let's let scan_folios() continue scanning the
second-oldest generation directly.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ce027c271e9b..31947fa60f18 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4718,6 +4718,7 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	int skipped = 0;
 	unsigned long remaining = nr_to_scan;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	unsigned long min_seq = lrugen->min_seq[type];
 
 	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
 	VM_WARN_ON_ONCE(!list_empty(list));
@@ -4725,8 +4726,8 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
 		return 0;
 
-	gen = lru_gen_from_seq(lrugen->min_seq[type]);
-
+next_gen:
+	gen = lru_gen_from_seq(min_seq);
 	for (i = MAX_NR_ZONES; i > 0; i--) {
 		LIST_HEAD(moved);
 		int skipped_zone = 0;
@@ -4768,6 +4769,14 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			break;
 	}
 
+	/*
+	 * This generation is exhausted across all zones, but the scan
+	 * target has not been reached yet. Continue with the next
+	 * reclaimable generation.
+	 */
+	if (i == 0 && ++min_seq + MIN_NR_GENS <= lrugen->max_seq)
+		goto next_gen;
+
 	item = PGSCAN_KSWAPD + reclaimer_offset(sc);
 	mod_lruvec_state(lruvec, item, isolated);
 	mod_lruvec_state(lruvec, PGREFILL, sorted);
-- 
2.34.1