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 __folio_migrate_mapping(), the rcu read lock is employed to safeguard
against the release of the memory cgroup in folio_migrate_mapping().

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>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/migrate.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 1bf2cf8c44dd4..45ba49af2136c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -672,6 +672,7 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 		struct lruvec *old_lruvec, *new_lruvec;
 		struct mem_cgroup *memcg;
 
+		rcu_read_lock();
 		memcg = folio_memcg(folio);
 		old_lruvec = mem_cgroup_lruvec(memcg, oldzone->zone_pgdat);
 		new_lruvec = mem_cgroup_lruvec(memcg, newzone->zone_pgdat);
@@ -699,6 +700,7 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 			mod_lruvec_state(new_lruvec, NR_FILE_DIRTY, nr);
 			__mod_zone_page_state(newzone, NR_ZONE_WRITE_PENDING, nr);
 		}
+		rcu_read_unlock();
 	}
 	local_irq_enable();
 
-- 
2.20.1