[RFC PATCH v2 1/5] mm: mglru: avoid scanning empty generations in scan_folios()

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

Commit 16b475d2ac3c ("mm/mglru: avoid reclaim type fall back when
isolation makes no progress") only falls back to the other type when
scanned == 0. However, I have frequently observed cases where
scanned > 0, but the older reclaimable generation becomes empty
after the first scan_folios(). As a result, the second
scan_folios() for the same type performs a redundant scan over an
empty generation.

We can avoid this by checking whether the reclaimable generation has
become empty when scanned < nr_to_scan and we still have fewer than
MIN_LRU_BATCH isolated folios after scan_folios().

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 566c4e837c7d..babbce4bbfe8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4852,11 +4852,14 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			break;
 		}
 		/*
-		 * If scanned > 0 and isolated == 0, avoid falling back to the
-		 * other type, as this type remains sufficient. Falling back
-		 * too readily can disrupt the positive_ctrl_err() bias.
+		 * If scanned >= nr_to_scan or isolated >= MIN_LRU_BATCH,
+		 * avoid falling back to the other type. The preferred
+		 * type is still reclaimable; otherwise, it would have
+		 * already run out of reclaimable generations. Falling
+		 * back too readily can disrupt the positive_ctrl_err()
+		 * bias.
 		 */
-		if (!scanned)
+		if (scanned < nr_to_scan && *isolated < MIN_LRU_BATCH)
 			type = !type;
 	}
 
-- 
2.39.3 (Apple Git-146)