[PATCH v6 18/33] mm: zswap: prevent memory cgroup release in zswap_compress()

[Qi Zheng <qi.zheng@linux.dev>]

From: Qi Zheng <zhengqi.arch@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 zswap_compress().

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
---
 mm/zswap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c
index a399f7a108304..fb525874a1b6b 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -893,11 +893,14 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry,
 	 * to the active LRU list in the case.
 	 */
 	if (comp_ret || !dlen || dlen >= PAGE_SIZE) {
+		rcu_read_lock();
 		if (!mem_cgroup_zswap_writeback_enabled(
 					folio_memcg(page_folio(page)))) {
+			rcu_read_unlock();
 			comp_ret = comp_ret ? comp_ret : -EINVAL;
 			goto unlock;
 		}
+		rcu_read_unlock();
 		comp_ret = 0;
 		dlen = PAGE_SIZE;
 		dst = kmap_local_page(page);
-- 
2.20.1