mm: multi-gen LRU: avoid futile retries

Recall that the per-node memcg LRU has two generations and they alternate
when the last memcg (of a given node) is moved from one to the other. 
Each generation is also sharded into multiple bins to improve scalability.
A reclaimer starts with a random bin (in the old generation) and, if it
fails, it will retry, i.e., to try the rest of the bins.

If a reclaimer fails with the last memcg, it should move this memcg to the
young generation first, which causes the generations to alternate, and
then retry.  Otherwise, the retries will be futile because all other bins
are empty.

Link: https://lkml.kernel.org/r/20230213075322.1416966-1-yuzhao@google.com
Fixes: e4dde56c ("mm: multi-gen LRU: per-node lru_gen_folio lists")
Signed-off-by: Yu Zhao <yuzhao@google.com>
Reported-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -5356,18 +5356,20 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
 
 static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
 {
+	int op;
 	int gen;
 	int bin;
 	int first_bin;
 	struct lruvec *lruvec;
 	struct lru_gen_folio *lrugen;
+	struct mem_cgroup *memcg;
 	const struct hlist_nulls_node *pos;
-	int op = 0;
-	struct mem_cgroup *memcg = NULL;
 	unsigned long nr_to_reclaim = get_nr_to_reclaim(sc);
 
 	bin = first_bin = get_random_u32_below(MEMCG_NR_BINS);
 restart:
+	op = 0;
+	memcg = NULL;
 	gen = get_memcg_gen(READ_ONCE(pgdat->memcg_lru.seq));
 
 	rcu_read_lock();
@@ -5391,14 +5393,22 @@ static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
 
 		op = shrink_one(lruvec, sc);
 
-		if (sc->nr_reclaimed >= nr_to_reclaim)
-			goto success;
-
 		rcu_read_lock();
+
+		if (sc->nr_reclaimed >= nr_to_reclaim)
+			break;
 	}
 
 	rcu_read_unlock();
 
+	if (op)
+		lru_gen_rotate_memcg(lruvec, op);
+
+	mem_cgroup_put(memcg);
+
+	if (sc->nr_reclaimed >= nr_to_reclaim)
+		return;
+
 	/* restart if raced with lru_gen_rotate_memcg() */
 	if (gen != get_nulls_value(pos))
 		goto restart;
@@ -5407,11 +5417,6 @@ static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
 	bin = get_memcg_bin(bin + 1);
 	if (bin != first_bin)
 		goto restart;
-success:
-	if (op)
-		lru_gen_rotate_memcg(lruvec, op);
-
-	mem_cgroup_put(memcg);
 }
 
 static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)