[PATCH] mm/mglru: Avoid falling back to another type when scan_folios() leaves no remaining pages

[Barry Song (Xiaomi)]

While remaining reaches 0 in scan_folios(), we quickly fall back
to the other type in isolate_folios(). This is incorrect, as the
current type may still have sufficient folios. Falling back can
undermine the positive_ctrl_err() result from get_type_to_scan(),
which is derived from swappiness.

A simple fix is to continue scanning this type for another round.

Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
---
 mm/vmscan.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ef45f3a4aa38..169fbbe17c7b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4788,8 +4788,13 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			*isolate_scanned = scanned;
 			break;
 		}
-
-		type = !type;
+		/*
+		 * 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)
+			type = !type;
 	}
 
 	return total_scanned;
-- 
2.34.1

Thanks
Barry