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

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: b5ba474 ("zswap: shrink zswap pool based on memory pressure")
Signed-off-by: Johannes Weiner <[email protected]>
Reported-by: Christian Heusel <[email protected]>
Debugged-by: Nhat Pham <[email protected]>
Suggested-by: Nhat Pham <[email protected]>
Tested-by: Christian Heusel <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Richard W.M. Jones <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Vitaly Wool <[email protected]>
Cc: <[email protected]>	[v6.8]
Signed-off-by: Andrew Morton <[email protected]>

Author: hnaz

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
-	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;