[PATCH 1/2] mm/zswap: Fix global shrinker when memory cgroup is disabled

[Hao Jia <jiahao.kernel@gmail.com>]

From: Hao Jia <jiahao1@lixiang.com>

When memory cgroup is disabled, mem_cgroup_iter() always returns NULL.
Therefore, the global shrinker shrink_worker() always takes the !memcg
branch. After MAX_RECLAIM_RETRIES empty walks, the worker simply gives up,
so it fails to write back anything.

Therefore, when memory cgroup is disabled, fall through with the !memcg
branch and shrink the root memcg directly.

With memcg disabled, shrink_memcg() only returns -ENOENT when the root
LRU is empty, which means the total pages are already below thr. The
loop then safely bails out via the zswap_total_pages() <= thr check.
For any other return value from shrink_memcg(), the loop is guaranteed
to terminate, either after MAX_RECLAIM_RETRIES failures or once the
threshold is met.

Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
Cc: stable@vger.kernel.org
Suggested-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosry@kernel.org>
Reported-by: Yosry Ahmed <yosry@kernel.org>
Closes: https://lore.kernel.org/all/CAO9r8zPVzMKFbCixxD-qgtRrkFxWVrHiZZeLc=eyTPKPVQgX4g@mail.gmail.com
Signed-off-by: Hao Jia <jiahao1@lixiang.com>
---
 mm/zswap.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index b5a17ea20237..3d697a1a5365 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1356,11 +1356,12 @@ static void shrink_worker(struct work_struct *w)
 		} while (memcg && !mem_cgroup_tryget_online(memcg));
 		spin_unlock(&zswap_shrink_lock);
 
-		if (!memcg) {
-			/*
-			 * Continue shrinking without incrementing failures if
-			 * we found candidate memcgs in the last tree walk.
-			 */
+		/*
+		 * A NULL memcg ends a full hierarchy pass (except when memcg is
+		 * disabled, where it is always NULL: fall through to the root LRU).
+		 * Count a failure only if the last pass found no candidates.
+		 */
+		if (!memcg && !mem_cgroup_disabled()) {
 			if (!attempts && ++failures == MAX_RECLAIM_RETRIES)
 				break;
 
-- 
2.34.1