FAILED: patch "[PATCH] mm: zswap: fix shrinker NULL crash with cgroup_disable=memory" failed to apply to 6.8-stable tree

The patch below does not apply to the 6.8-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.8.y
git checkout FETCH_HEAD
git cherry-pick -x 682886ec69d22363819a83ddddd5d66cb5c791e1
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024042923-monday-hamlet-26ca@gregkh' --subject-prefix 'PATCH 6.8.y' HEAD^..

Possible dependencies:

682886ec69d2 ("mm: zswap: fix shrinker NULL crash with cgroup_disable=memory") 30fb6a8d9e33 ("mm: zswap: fix writeback shinker GFP_NOIO/GFP_NOFS recursion") e35606e4167d ("mm/zswap: global lru and shrinker shared by all zswap_pools fix") bf9b7df23cb3 ("mm/zswap: global lru and shrinker shared by all zswap_pools") 5182661a11ba ("mm: zswap: function ordering: move entry sections out of LRU section") 506a86c5e221 ("mm: zswap: function ordering: public lru api") abca07c04aa5 ("mm: zswap: function ordering: pool params")
c1a0ecb82bdc ("mm: zswap: function ordering: zswap_pools")
39f3ec8eaa60 ("mm: zswap: function ordering: pool refcounting") a984649b5c1f ("mm: zswap: function ordering: pool alloc & free") be7fc97c5283 ("mm: zswap: further cleanup zswap_store()")
fa9ad6e21003 ("mm: zswap: break out zwap_compress()")
ff2972aa1b5d ("mm: zswap: rename __zswap_load() to zswap_decompress()") 7dd1f7f0fc1c ("mm: zswap: move zswap_invalidate_entry() to related functions") 5b297f70bb26 ("mm: zswap: inline and remove zswap_entry_find_get()") 5878303c5353 ("mm/zswap: fix race between lru writeback and swapoff") db128f5fdee9 ("mm: zswap: remove unused tree argument in zswap_entry_put()") 44c7c734a513 ("mm/zswap: split zswap rb-tree")
bb29fd7760ae ("mm/zswap: make sure each swapfile always have zswap rb-tree") 8409a385a6b4 ("mm/zswap: improve with alloc_workqueue() call")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 682886ec69d22363819a83ddddd5d66cb5c791e1 Mon Sep 17 00:00:00 2001
From: Johannes Weiner <[email protected]>
Date: Thu, 18 Apr 2024 08:26:28 -0400
Subject: [PATCH] mm: zswap: fix shrinker NULL crash with cgroup_disable=memory

Christian reports a NULL deref in zswap that he bisected down to the zswap shrinker. The issue also cropped up in the bug trackers of libguestfs [1] and the Red Hat bugzilla [2].

The problem is that when memcg is disabled with the boot time flag, the zswap shrinker might get called with sc->memcg == NULL. This is okay in many places, like the lruvec operations. But it crashes in memcg_page_state() - which is only used due to the non-node accounting of cgroup's the zswap memory to begin with.

Nhat spotted that the memcg can be NULL in the memcg-disabled case, and I was then able to reproduce the crash locally as well.

[1] https://github.com/libguestfs/libguestfs/issues/139
[2] https://bugzilla.redhat.com/show_bug.cgi?id=2275252

index caed028945b0..6f8850c44b61 100644
@@ -1331,15 +1331,22 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 	if (!gfp_has_io_fs(sc->gfp_mask))
 		return 0;
 
-#ifdef CONFIG_MEMCG_KMEM
-	mem_cgroup_flush_stats(memcg);
-	nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
-	nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
-#else
-	/* use pool stats instead of memcg stats */
-	nr_backing = zswap_pool_total_size >> PAGE_SHIFT;
-	nr_stored = atomic_read(&zswap_nr_stored);
-#endif
+	/*
+	 * For memcg, use the cgroup-wide ZSWAP stats since we don't
+	 * have them per-node and thus per-lruvec. Careful if memcg is
+	 * runtime-disabled: we can get sc->memcg == NULL, which is ok
+	 * for the lruvec, but not for memcg_page_state().
+	 *
+	 * Without memcg, use the zswap pool-wide metrics.
+	 */
+	if (!mem_cgroup_disabled()) {
+		mem_cgroup_flush_stats(memcg);
+		nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
+		nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
+	} else {
+		nr_backing = zswap_pool_total_size >> PAGE_SHIFT;
+		nr_stored = atomic_read(&zswap_nr_stored);
+	}
 
 	if (!nr_stored)
 		return 0;

Christian reports a NULL deref in zswap that he bisected down to the zswap shrinker. The issue also cropped up in the bug trackers of libguestfs [1] and the Red Hat bugzilla [2].

The problem is that when memcg is disabled with the boot time flag, the zswap shrinker might get called with sc->memcg == NULL. This is okay in many places, like the lruvec operations. But it crashes in memcg_page_state() - which is only used due to the non-node accounting of cgroup's the zswap memory to begin with.

Nhat spotted that the memcg can be NULL in the memcg-disabled case, and I was then able to reproduce the crash locally as well.

[1] https://github.com/libguestfs/libguestfs/issues/139
[2] https://bugzilla.redhat.com/show_bug.cgi?id=2275252

---
 mm/zswap.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

Two minor conflicts in the else branch:
- zswap_pool_total_size was get_zswap_pool_size() in 6.8
- zswap_nr_stored was pool->nr_stored in 6.8

index e9c04387195f..69766f2c5a6c 100644
@@ -653,15 +653,22 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 	if (!gfp_has_io_fs(sc->gfp_mask))
 		return 0;
 
-#ifdef CONFIG_MEMCG_KMEM
-	mem_cgroup_flush_stats(memcg);
-	nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
-	nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
-#else
-	/* use pool stats instead of memcg stats */
-	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
-	nr_stored = atomic_read(&pool->nr_stored);
-#endif
+	/*
+	 * For memcg, use the cgroup-wide ZSWAP stats since we don't
+	 * have them per-node and thus per-lruvec. Careful if memcg is
+	 * runtime-disabled: we can get sc->memcg == NULL, which is ok
+	 * for the lruvec, but not for memcg_page_state().
+	 *
+	 * Without memcg, use the zswap pool-wide metrics.
+	 */
+	if (!mem_cgroup_disabled()) {
+		mem_cgroup_flush_stats(memcg);
+		nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
+		nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
+	} else {
+		nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
+		nr_stored = atomic_read(&pool->nr_stored);
+	}
 
 	if (!nr_stored)
 		return 0;

2.44.0

Now queued up, thanks.

greg k-h