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

Link: https://lkml.kernel.org/r/20240418124043.GC1055428@cmpxchg.org
Link: https://lkml.kernel.org/r/20240417143324.GA1055428@cmpxchg.org
Fixes: b5ba474f ("zswap: shrink zswap pool based on memory pressure")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Christian Heusel <christian@heusel.eu>
Debugged-by: Nhat Pham <nphamcs@gmail.com>
Suggested-by: Nhat Pham <nphamcs@gmail.com>
Tested-by: Christian Heusel <christian@heusel.eu>
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Richard W.M. Jones <rjones@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: <stable@vger.kernel.org>	[v6.8]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zswap.c

@@ -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
+	/*
+	 * 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
-	/* use pool stats instead of memcg stats */
+	} else {
 		nr_backing = zswap_pool_total_size >> PAGE_SHIFT;
 		nr_stored = atomic_read(&zswap_nr_stored);
-#endif
+	}
 
 	if (!nr_stored)
 		return 0;