linux-kernel - [PATCH] mm/mglru: Stop try_to_inc_min_seq() if the oldest generation LRU lists are not empty

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Message-Id: <20250630080603.36171-1-jiahao.kernel@gmail.com>
Date: Mon, 30 Jun 2025 16:06:03 +0800
From: Hao Jia <jiahao.kernel@...il.com>
To: akpm@...ux-foundation.org,
	hannes@...xchg.org,
	yuzhao@...gle.com,
	kinseyho@...gle.com,
	david@...hat.com,
	mhocko@...nel.org,
	zhengqi.arch@...edance.com,
	shakeel.butt@...ux.dev,
	lorenzo.stoakes@...cle.com
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Hao Jia <jiahao1@...iang.com>
Subject: [PATCH] mm/mglru: Stop try_to_inc_min_seq() if the oldest generation LRU lists are not empty

From: Hao Jia <jiahao1@...iang.com>

In try_to_inc_min_seq(), if the oldest generation of LRU lists
(anonymous and file) are not empty. Then we should return directly
to avoid unnecessary subsequent overhead.

Corollary: If the lrugen->folios[gen][type][zone] lists of both
anonymous and file are not empty, try_to_inc_min_seq() will fail.

Proof: Taking LRU_GEN_ANON as an example, consider the following two cases:

Case 1: min_seq[LRU_GEN_ANON] <= seq (seq is lrugen->max_seq - MIN_NR_GENS)

Since min_seq[LRU_GEN_ANON] has not increased,
so min_seq[LRU_GEN_ANON] is still equal to lrugen->min_seq[LRU_GEN_ANON].
Therefore, in the following judgment:
min_seq[LRU_GEN_ANON] <= lrugen->min_seq[LRU_GEN_ANON] is always true.
So, we will not increase the seq of the oldest generation of anonymous,
and try_to_inc_min_seq() will return false.

case 2: min_seq[LRU_GEN_ANON] > seq (seq is lrugen->max_seq - MIN_NR_GENS)

If min_seq[LRU_GEN_ANON] > seq, that is, lrugen->min_seq[LRU_GEN_ANON] > seq
Then min_seq[LRU_GEN_ANON] is assigned seq.
Therefore, in the following judgment:
min_seq[LRU_GEN_ANON] (seq) <= lrugen->min_seq[LRU_GEN_ANON] is always true.
So, we will not update the oldest generation seq of anonymous,
and try_to_inc_min_seq() will return false.

It is similar for LRU_GEN_FILE. Therefore, in try_to_inc_min_seq(),
if the oldest generation LRU lists (anonymous and file) are not empty,
in other words, min_seq[type] has not increased.
we can directly return false to avoid unnecessary checking overhead later.

Signed-off-by: Hao Jia <jiahao1@...iang.com>
---
 mm/vmscan.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f8dfd2864bbf..3ba63d87563f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3928,6 +3928,7 @@ static bool try_to_inc_min_seq(struct lruvec *lruvec, int swappiness)
 	int gen, type, zone;
 	bool success = false;
 	struct lru_gen_folio *lrugen = &lruvec->lrugen;
+	int seq_inc_flags[ANON_AND_FILE] = {0};
 	DEFINE_MIN_SEQ(lruvec);

 	VM_WARN_ON_ONCE(!seq_is_valid(lruvec));
@@ -3943,11 +3944,20 @@ static bool try_to_inc_min_seq(struct lruvec *lruvec, int swappiness)
 			}

 			min_seq[type]++;
+			seq_inc_flags[type] = 1;
 		}
 next:
 		;
 	}

+	/*
+	 * If the oldest generation of LRU lists (anonymous and file)
+	 * are not empty, we can directly return false to avoid unnecessary
+	 * checking overhead later.
+	 */
+	if (!seq_inc_flags[LRU_GEN_ANON] && !seq_inc_flags[LRU_GEN_FILE])
+		return success;
+
 	/* see the comment on lru_gen_folio */
 	if (swappiness && swappiness <= MAX_SWAPPINESS) {
 		unsigned long seq = lrugen->max_seq - MIN_NR_GENS;
-- 
2.34.1