[PATCH v3 08/30] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

[Qi Zheng <qi.zheng@linux.dev>]

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>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/memcontrol.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 982c9f5cf72cb..0458fc2e810ff 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -991,14 +991,18 @@ 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;
+	do {
+		memcg = folio_memcg(folio);
+	} while (unlikely(!css_tryget(&memcg->css)));
 	rcu_read_unlock();
 	return memcg;
 }
-- 
2.20.1