[PATCH v1 08/26] buffer: prevent memory cgroup release in folio_alloc_buffers()

Qi Zheng
2025-10-28T14:03:04Z

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>
---
fs/buffer.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 6a8752f7bbedb..bc93d0b1d0c30 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -925,8 +925,7 @@ 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;
@@ -947,6 +946,7 @@ 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