[PATCH v2 2/2] mm/mglru: make retry logic explicit in isolate_folios()

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

The existing mainline code retries the same type once in a rather
subtle way. `for_each_evictable_type()` may provide one more iteration,
allowing the same type to be retried if we scanned some folios but
failed to isolate any due to protections, promotions, or races. This
patch makes the retry behavior explicit.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 35a233623368..718f59ffc688 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4852,6 +4852,7 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	bool type_fallback_allowed = !is_single_type_reclaim(swappiness);
 	int type = get_type_to_scan(lruvec, swappiness);
 	int total_scanned = 0, scanned, tier;
+	bool tried = false;
 
 retry:
 	tier = get_tier_idx(lruvec, type);
@@ -4871,9 +4872,18 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	 */
 	if (!scanned && type_fallback_allowed) {
 		type = !type;
+		tried = true;
 		type_fallback_allowed = false;
 		goto retry;
 	}
+	/*
+	 * We scanned some folios but failed to isolate any due to promotions,
+	 * protections, or races. Retry once to avoid a larger loop.
+	 */
+	if (scanned && !tried) {
+		tried = true;
+		goto retry;
+	}
 
 	return total_scanned;
 }
-- 
2.34.1