From: Hao Jia <jiahao1@lixiang.com>
Date: Thu, 3 Jul 2025 02:39:46 +0000 (+0800)
Subject: mm/mglru: stop try_to_inc_min_seq() if min_seq[type] has not increased
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=526660b950a425b9d32798703b839e3c08cdf81b;p=thirdparty%2Flinux.git

mm/mglru: stop try_to_inc_min_seq() if min_seq[type] has not increased

In try_to_inc_min_seq(), if min_seq[type] has not increased.  In other
words, min_seq[type] == lrugen->min_seq[type].  Then we should return
directly to avoid unnecessary overhead later.

Corollary: If min_seq[type] of both anonymous and file is not increased,
try_to_inc_min_seq() will fail.

Proof:
It is known that min_seq[type] has not increased, that is, min_seq[type]
is equal to lrugen->min_seq[type], then the following:

case 1: min_seq[type] has not been reassigned and changed before
judgment min_seq[type] <= lrugen->min_seq[type].
Then the subsequent min_seq[type] <= lrugen->min_seq[type] judgment
will always be true.

case 2: min_seq[type] is reassigned to seq, before judgment
min_seq[type] <= lrugen->min_seq[type].
Then at least the condition of min_seq[type] > seq must be met
before min_seq[type] will be reassigned to seq.
That is to say, before the reassignment, lrugen->min_seq[type] > seq
is met, and then min_seq[type] = seq.
Then the following min_seq[type](seq) <= lrugen->min_seq[type] judgment
is always true.

Therefore, in try_to_inc_min_seq(), If min_seq[type] of both anonymous
and file is not increased, we can return false directly to avoid
unnecessary overhead.

Link: https://lkml.kernel.org/r/20250703023946.65315-1-jiahao.kernel@gmail.com
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
Suggested-by: Yuanchu Xie <yuanchu@google.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kinsey Ho <kinseyho@google.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 19bfce93b3730..0d6b8e1e95e50 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3921,6 +3921,7 @@ static bool try_to_inc_min_seq(struct lruvec *lruvec, int swappiness)
 {
 	int gen, type, zone;
 	bool success = false;
+	bool seq_inc_flag = false;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
 	DEFINE_MIN_SEQ(lruvec);
 
@@ -3937,11 +3938,20 @@ static bool try_to_inc_min_seq(struct lruvec *lruvec, int swappiness)
 			}
 
 			min_seq[type]++;
+			seq_inc_flag = true;
 		}
 next:
 		;
 	}
 
+	/*
+	 * If min_seq[type] of both anonymous and file is not increased,
+	 * we can directly return false to avoid unnecessary checking
+	 * overhead later.
+	 */
+	if (!seq_inc_flag)
+		return success;
+
 	/* see the comment on lru_gen_folio */
 	if (swappiness && swappiness <= MAX_SWAPPINESS) {
 		unsigned long seq = lrugen->max_seq - MIN_NR_GENS;