mm: vmscan: pass root_mem_cgroup instead of NULL to memcg aware shrinker

It's just convenient to implement a memcg aware shrinker when you know
that shrink_control->memcg != NULL unless memcg_kmem_enabled() returns
false.

Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -382,9 +382,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
  *
  * @memcg specifies the memory cgroup to target. If it is not NULL,
  * only shrinkers with SHRINKER_MEMCG_AWARE set will be called to scan
- * objects from the memory cgroup specified. Otherwise all shrinkers
- * are called, and memcg aware shrinkers are supposed to scan the
- * global list then.
+ * objects from the memory cgroup specified. Otherwise, only unaware
+ * shrinkers are called.
  *
  * @nr_scanned and @nr_eligible form a ratio that indicate how much of
  * the available objects should be scanned.  Page reclaim for example
@@ -404,7 +403,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 	struct shrinker *shrinker;
 	unsigned long freed = 0;
 
-	if (memcg && !memcg_kmem_online(memcg))
+	if (memcg && (!memcg_kmem_enabled() || !mem_cgroup_online(memcg)))
 		return 0;
 
 	if (nr_scanned == 0)
@@ -428,7 +427,13 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 			.memcg = memcg,
 		};
 
-		if (memcg && !(shrinker->flags & SHRINKER_MEMCG_AWARE))
+		/*
+		 * If kernel memory accounting is disabled, we ignore
+		 * SHRINKER_MEMCG_AWARE flag and call all shrinkers
+		 * passing NULL for memcg.
+		 */
+		if (memcg_kmem_enabled() &&
+		    !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
 			continue;
 
 		if (!(shrinker->flags & SHRINKER_NUMA_AWARE))