[RFC PATCH v3 4/6] mm: mglru: improve scan_folios() exhaustion detection

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

Commit 16b475d2ac3c ("mm/mglru: avoid reclaim type fall back when
isolation makes no progress") uses scanned == 0 to determine
whether scan_folios() has exhausted a reclaim type. However,
this is not always sufficient. It is possible for scanned > 0,
while the oldest reclaimable generation is exhausted after the
first scan_folios() call.

Add an exhausted output argument to scan_folios() so it can
explicitly report whether the reclaimable lists for the current
type have been exhausted.

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 mm/vmscan.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0038f33aa318..2e7fef6975b6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4707,7 +4707,8 @@ static bool isolate_folio(struct lruvec *lruvec, struct folio *folio, struct sca
 
 static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 		       struct scan_control *sc, int type, int tier,
-		       struct list_head *list, int *isolatedp)
+		       struct list_head *list, int *isolatedp,
+		       bool *exhausted)
 {
 	int i;
 	int gen;
@@ -4723,8 +4724,10 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	VM_WARN_ON_ONCE(nr_to_scan > MAX_LRU_BATCH);
 	VM_WARN_ON_ONCE(!list_empty(list));
 
-	if (get_nr_gens(lruvec, type) == MIN_NR_GENS)
+	if (get_nr_gens(lruvec, type) == MIN_NR_GENS) {
+		*exhausted = true;
 		return 0;
+	}
 
 next_gen:
 	gen = lru_gen_from_seq(min_seq);
@@ -4785,6 +4788,11 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 				scanned, skipped, isolated,
 				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
 
+	/*
+	 * This scan exhausted the reclaimable lists before reaching the
+	 * scan target or accumulating enough isolated folios.
+	 */
+	*exhausted = remaining > 0 && isolated < MIN_LRU_BATCH;
 	*isolatedp = isolated;
 	return scanned;
 }
@@ -4842,12 +4850,12 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	bool single_type = is_single_type_reclaim(swappiness);
 	int type = get_type_to_scan(lruvec, swappiness);
 	int total_scanned = 0, scanned, tier;
-	bool tried = false;
+	bool tried = false, exhausted;
 
 retry:
 	tier = get_tier_idx(lruvec, type);
 	scanned = scan_folios(nr_to_scan, lruvec, sc,
-			      type, tier, list, isolated);
+			      type, tier, list, isolated, &exhausted);
 
 	total_scanned += scanned;
 	if (*isolated) {
@@ -4860,7 +4868,7 @@ static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 	 * We are running out of the current reclaim type. Fall back to
 	 * the other type if allowed.
 	 */
-	if (!tried && !scanned && !single_type) {
+	if (!tried && exhausted && !single_type) {
 		type = !type;
 		tried = true;
 		goto retry;
-- 
2.34.1