mm: vmscan: add shrinker_info_protected() helper

The shrinker_info is dereferenced in a couple of places via
rcu_dereference_protected with different calling conventions, for
example, using mem_cgroup_nodeinfo helper or dereferencing
memcg->nodeinfo[nid]->shrinker_info.  And the later patch will add more
dereference places.

So extract the dereference into a helper to make the code more readable.
No functional change.

[akpm@linux-foundation.org: retain rcu_dereference_protected() in free_shrinker_info(), per Hugh]

Link: https://lkml.kernel.org/r/20210311190845.9708-8-shy828301@gmail.comSigned-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -192,6 +192,13 @@ static inline int shrinker_map_size(int nr_items)
 	return (DIV_ROUND_UP(nr_items, BITS_PER_LONG) * sizeof(unsigned long));
 }
 
+static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
+						     int nid)
+{
+	return rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
+					 lockdep_is_held(&shrinker_rwsem));
+}
+
 static int expand_one_shrinker_info(struct mem_cgroup *memcg,
 				    int size, int old_size)
 {
@@ -201,7 +208,7 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
 
 	for_each_node(nid) {
 		pn = memcg->nodeinfo[nid];
-		old = rcu_dereference_protected(pn->shrinker_info, true);
+		old = shrinker_info_protected(memcg, nid);
 		/* Not yet online memcg */
 		if (!old)
 			return 0;
@@ -675,8 +682,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 	if (!down_read_trylock(&shrinker_rwsem))
 		return 0;
 
-	info = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_info,
-					 true);
+	info = shrinker_info_protected(memcg, nid);
 	if (unlikely(!info))
 		goto unlock;