[PATCH RFC 18/28] mm: memcontrol: prevent memory cgroup release in mem_cgroup_swap_full()

[Muchun Song]

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 mem_cgroup_swap_full().

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

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 09ecb5cb78f2..694f19017699 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5097,17 +5097,21 @@ bool mem_cgroup_swap_full(struct folio *folio)
 	if (do_memsw_account())
 		return false;
 
-	memcg = folio_memcg(folio);
-	if (!memcg)
+	if (!folio_memcg_charged(folio))
 		return false;
 
+	rcu_read_lock();
+	memcg = folio_memcg(folio);
 	for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
 		unsigned long usage = page_counter_read(&memcg->swap);
 
 		if (usage * 2 >= READ_ONCE(memcg->swap.high) ||
-		    usage * 2 >= READ_ONCE(memcg->swap.max))
+		    usage * 2 >= READ_ONCE(memcg->swap.max)) {
+			rcu_read_unlock();
 			return true;
+		}
 	}
+	rcu_read_unlock();
 
 	return false;
 }
-- 
2.20.1