[PATCH v4 09/31] buffer: prevent memory cgroup release in folio_alloc_buffers()

[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 function get_mem_cgroup_from_folio() is
employed to safeguard against the release of the memory cgroup.
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>
---
 fs/buffer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index b67791690ed33..d80b635cff162 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -926,8 +926,7 @@ struct buffer_head *folio_alloc_buffers(struct folio *folio, unsigned long size,
 	long offset;
 	struct mem_cgroup *memcg, *old_memcg;
 
-	/* The folio lock pins the memcg */
-	memcg = folio_memcg(folio);
+	memcg = get_mem_cgroup_from_folio(folio);
 	old_memcg = set_active_memcg(memcg);
 
 	head = NULL;
@@ -948,6 +947,7 @@ struct buffer_head *folio_alloc_buffers(struct folio *folio, unsigned long size,
 	}
 out:
 	set_active_memcg(old_memcg);
+	mem_cgroup_put(memcg);
 	return head;
 /*
  * In case anything failed, we just free everything we got.
-- 
2.20.1