[RFC PATCH v3 3/6] mm/mglru: improve readability of isolate_folios()

[Barry Song (Xiaomi)]

From: Ridong Chen <chenridong@xxxxxxxxxx>

The for_each_evictable_type() loop in isolate_folios()
is misleading: it does not actually iterate over each
evictable type. Instead, get_type_to_scan() selects the
type to scan, while the iterator `i` merely bounds the
number of attempts.

Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
Co-developed-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
---
 mm/vmscan.c | 47 +++++++++++++++++++++++++++--------------------
 1 file changed, 27 insertions(+), 20 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 31947fa60f18..0038f33aa318 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4828,35 +4828,42 @@ static int get_type_to_scan(struct lruvec *lruvec, int swappiness)
 	return positive_ctrl_err(&sp, &pv);
 }
 
+static inline bool is_single_type_reclaim(int swappiness)
+{
+	return swappiness == MIN_SWAPPINESS ||
+	       swappiness == SWAPPINESS_ANON_ONLY;
+}
+
 static int isolate_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
 			  struct scan_control *sc, int swappiness,
 			  struct list_head *list, int *isolated,
 			  int *isolate_type, int *isolate_scanned)
 {
-	int i;
-	int total_scanned = 0;
+	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;
 
-	for_each_evictable_type(i, swappiness) {
-		int scanned;
-		int tier = get_tier_idx(lruvec, type);
+retry:
+	tier = get_tier_idx(lruvec, type);
+	scanned = scan_folios(nr_to_scan, lruvec, sc,
+			      type, tier, list, isolated);
 
-		scanned = scan_folios(nr_to_scan, lruvec, sc,
-				      type, tier, list, isolated);
+	total_scanned += scanned;
+	if (*isolated) {
+		*isolate_type = type;
+		*isolate_scanned = scanned;
+		return total_scanned;
+	}
 
-		total_scanned += scanned;
-		if (*isolated) {
-			*isolate_type = type;
-			*isolate_scanned = scanned;
-			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)
-			type = !type;
+	/*
+	 * We are running out of the current reclaim type. Fall back to
+	 * the other type if allowed.
+	 */
+	if (!tried && !scanned && !single_type) {
+		type = !type;
+		tried = true;
+		goto retry;
 	}
 
 	return total_scanned;
-- 
2.34.1