[PATCH v5 1/6] 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. Stop the loop once
shrink_memcg() reports -ENOENT, since the root LRU is the only target and
-ENOENT means it has been exhausted.

Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
Cc: stable@vger.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 | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 761cd699e0a3..0f8f04f22888 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1356,7 +1356,12 @@ static void shrink_worker(struct work_struct *w)
 		} while (memcg && !mem_cgroup_tryget_online(memcg));
 		spin_unlock(&zswap_shrink_lock);
 
-		if (!memcg) {
+		/*
+		 * Reaching a NULL memcg means a full hierarchy pass completed.
+		 * Exclude the memcg-disabled case, where it is always NULL, and
+		 * fall through to shrink the root LRU directly.
+		 */
+		if (!memcg && !mem_cgroup_disabled()) {
 			/*
 			 * Continue shrinking without incrementing failures if
 			 * we found candidate memcgs in the last tree walk.
@@ -1378,8 +1383,15 @@ static void shrink_worker(struct work_struct *w)
 		 * with pages in zswap. Skip this without incrementing attempts
 		 * and failures.
 		 */
-		if (ret == -ENOENT)
+		if (ret == -ENOENT) {
+			/*
+			 * With memcg disabled the root LRU is the only target, so
+			 * we should abort if it has no writeback-candidate pages.
+			 */
+			if (mem_cgroup_disabled())
+				break;
 			continue;
+		}
 		++attempts;
 
 		if (ret && ++failures == MAX_RECLAIM_RETRIES)
-- 
2.34.1