[PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

Qi Zheng
2025-10-28T14:03:00Z

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 get_mem_cgroup_from_folio().

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>
---
mm/memcontrol.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d484b632c790f..1da3ad77054d3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
*/
struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
{
- struct mem_cgroup *memcg = folio_memcg(folio);
+ struct mem_cgroup *memcg;

if (mem_cgroup_disabled())
return NULL;

+ if (!folio_memcg_charged(folio))
+ return root_mem_cgroup;
+
rcu_read_lock();
- if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
- memcg = root_mem_cgroup;
+retry:
+ memcg = folio_memcg(folio);
+ if (unlikely(!css_tryget(&memcg->css)))
+ goto retry;
rcu_read_unlock();
return memcg;
}
--
2.20.1