[PATCH v3 1/2] mm/zswap: Fix global shrinker when memory cgroup is disabled

[Hao Jia]

From: Hao Jia <jiahao1@xxxxxxxxxxx>

Zswap writeback when the global pool limit is hit fails when memory
cgroup is disabled. The pool remains full until it is organically
drained by swapins or memory freeing, leading to zswap store failures
and pages bypassing getting written directly to the backing swap device,
causing LRU inversion (hotter pages with higher fault latency).

This happens because mem_cgroup_iter() always returns NULL when
memory cgroups are disabled. As a result, the global shrinker
shrink_worker() repeatedly takes empty walks. After MAX_RECLAIM_RETRIES
failed attempts, the worker gives up without writing back any pages.

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. In the
absence of heavy concurrent zswap stores, the loop then safely bails out
via the zswap_total_pages() <= thr check; otherwise, it will resume
shrinking the memcg after processing the reschedule 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@xxxxxxxxxxxxxxx
Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>
Acked-by: Yosry Ahmed <yosry@xxxxxxxxxx>
Reported-by: Yosry Ahmed <yosry@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/CAO9r8zPVzMKFbCixxD-qgtRrkFxWVrHiZZeLc=eyTPKPVQgX4g@xxxxxxxxxxxxxx
Signed-off-by: Hao Jia <jiahao1@xxxxxxxxxxx>
---
 mm/zswap.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index b5a17ea20237..48fc7b575e24 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;
 
@@ -1379,7 +1380,7 @@ static void shrink_worker(struct work_struct *w)
 		 * and failures.
 		 */
 		if (ret == -ENOENT)
-			continue;
+			goto resched;
 		++attempts;
 
 		if (ret && ++failures == MAX_RECLAIM_RETRIES)
-- 
2.34.1