[PATCH v1 10/26] mm: memcontrol: prevent memory cgroup release in count_memcg_folio_events()

Qi Zheng
2025-10-28T14:03:21Z

From: Muchun Song <songmuchun@bytedance.com>

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@bytedance.com>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.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 174e52d8e7039..ca8d4e09cbe7d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -984,10 +984,15 @@ void count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
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