linux-kernel - [PATCH RFC 02/28] mm: memcontrol: use folio_memcg_charged() to avoid potential rcu lock holding

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Message-Id: <20250415024532.26632-3-songmuchun@bytedance.com>
Date: Tue, 15 Apr 2025 10:45:06 +0800
From: Muchun Song <songmuchun@...edance.com>
To: hannes@...xchg.org,
	mhocko@...nel.org,
	roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev,
	muchun.song@...ux.dev,
	akpm@...ux-foundation.org,
	david@...morbit.com,
	zhengqi.arch@...edance.com,
	yosry.ahmed@...ux.dev,
	nphamcs@...il.com,
	chengming.zhou@...ux.dev
Cc: linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org,
	linux-mm@...ck.org,
	hamzamahfooz@...ux.microsoft.com,
	apais@...ux.microsoft.com,
	Muchun Song <songmuchun@...edance.com>
Subject: [PATCH RFC 02/28] mm: memcontrol: use folio_memcg_charged() to avoid potential rcu lock holding

If a folio isn't charged to the memory cgroup, holding an rcu read lock
is needless. Users only want to know its charge status, so use
folio_memcg_charged() here.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 mm/memcontrol.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 61488e45cab2..0fc76d50bc23 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -797,20 +797,17 @@ void __mod_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 void __lruvec_stat_mod_folio(struct folio *folio, enum node_stat_item idx,
 			     int val)
 {
-	struct mem_cgroup *memcg;
 	pg_data_t *pgdat = folio_pgdat(folio);
 	struct lruvec *lruvec;
 
-	rcu_read_lock();
-	memcg = folio_memcg(folio);
-	/* Untracked pages have no memcg, no lruvec. Update only the node */
-	if (!memcg) {
-		rcu_read_unlock();
+	if (!folio_memcg_charged(folio)) {
+		/* Untracked pages have no memcg, no lruvec. Update only the node */
 		__mod_node_page_state(pgdat, idx, val);
 		return;
 	}
 
-	lruvec = mem_cgroup_lruvec(memcg, pgdat);
+	rcu_read_lock();
+	lruvec = mem_cgroup_lruvec(folio_memcg(folio), pgdat);
 	__mod_lruvec_state(lruvec, idx, val);
 	rcu_read_unlock();
 }
-- 
2.20.1