[PATCH v4 15/31] mm: memcontrol: prevent memory cgroup release in mem_cgroup_swap_full()

[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 mem_cgroup_swap_full().

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/memcontrol.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5508a4aced0cc..641c2fa077ccf 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5283,27 +5283,29 @@ long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
 bool mem_cgroup_swap_full(struct folio *folio)
 {
 	struct mem_cgroup *memcg;
+	bool ret = false;
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
 	if (vm_swap_full())
 		return true;
-	if (do_memsw_account())
-		return false;
+	if (do_memsw_account() || !folio_memcg_charged(folio))
+		return ret;
 
+	rcu_read_lock();
 	memcg = folio_memcg(folio);
-	if (!memcg)
-		return false;
-
 	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))
-			return true;
+		    usage * 2 >= READ_ONCE(memcg->swap.max)) {
+			ret = true;
+			break;
+		}
 	}
+	rcu_read_unlock();
 
-	return false;
+	return ret;
 }
 
 static int __init setup_swap_account(char *s)
-- 
2.20.1