+ mm-zswap-fix-pool-refcount-bug-around-shrink_worker.patch added to mm-hotfixes-unstable branch

+ mm-zswap-fix-pool-refcount-bug-around-shrink_worker.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: zswap: fix pool refcount bug around shrink_worker()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-zswap-fix-pool-refcount-bug-around-shrink_worker.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-zswap-fix-pool-refcount-bug-around-shrink_worker.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: mm: zswap: fix pool refcount bug around shrink_worker()
Date: Fri, 6 Oct 2023 12:00:24 -0400

When a zswap store fails due to the limit, it acquires a pool reference
and queues the shrinker.  When the shrinker runs, it drops the reference. 
However, there can be multiple store attempts before the shrinker wakes up
and runs once.  This results in reference leaks and eventual saturation
warnings for the pool refcount.

Fix this by dropping the reference again when the shrinker is already
queued.  This ensures one reference per shrinker run.

Link: https://lkml.kernel.org/r/20231006160024.170748-1-hannes@cmpxchg.org
Fixes: 45190f01dd40 ("mm/zswap.c: add allocation hysteresis if pool limit is hit")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Chris Mason <clm@fb.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: <stable@vger.kernel.org>	[5.6+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/zswap.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/zswap.c~mm-zswap-fix-pool-refcount-bug-around-shrink_worker
+++ a/mm/zswap.c
@@ -1383,8 +1383,8 @@ reject:
 
 shrink:
 	pool = zswap_pool_last_get();
-	if (pool)
-		queue_work(shrink_wq, &pool->shrink_work);
+	if (pool && !queue_work(shrink_wq, &pool->shrink_work))
+		zswap_pool_put(pool);
 	goto reject;
 }
 
_

Patches currently in -mm which might be from hannes@cmpxchg.org are

mm-zswap-fix-pool-refcount-bug-around-shrink_worker.patch