linux-kernel - [PATCH RFC 14/28] mm: memcontrol: prevent memory cgroup release in count_memcg_folio

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Message-Id: <20250415024532.26632-15-songmuchun@bytedance.com>
Date: Tue, 15 Apr 2025 10:45:18 +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 14/28] mm: memcontrol: prevent memory cgroup release in count_memcg_folio_events()

In the near future, a folio will no longer pin its corresponding
memory cgroup. To ensure safety, it will only be appropriate to
hold the rcu read lock or acquire a reference to the memory cgroup
returned by folio_memcg(), thereby preventing it from being released.

In the current patch, the rcu read lock is employed to safeguard
against the release of the memory cgroup in count_memcg_folio_events().

This serves as a preparatory measure for the reparenting of the
LRU pages.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 include/linux/memcontrol.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index a9ef2087c735..01239147eb11 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -978,10 +978,15 @@ static inline void count_memcg_events(struct mem_cgroup *memcg,
 static inline void count_memcg_folio_events(struct folio *folio,
 		enum vm_event_item idx, unsigned long nr)
 {
-	struct mem_cgroup *memcg = folio_memcg(folio);
+	struct mem_cgroup *memcg;
 
-	if (memcg)
-		count_memcg_events(memcg, idx, nr);
+	if (!folio_memcg_charged(folio))
+		return;
+
+	rcu_read_lock();
+	memcg = folio_memcg(folio);
+	count_memcg_events(memcg, idx, nr);
+	rcu_read_unlock();
 }
 
 static inline void count_memcg_events_mm(struct mm_struct *mm,
-- 
2.20.1