[PATCH] memcg: Add comment to mem_cgroup_css_online()

[Kirill Tkhai]

Explain relationships between allocation and expanding.

Suggested-by: Vladimir Davydov <vdavydov....@gmail.com>
Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d90993ef1d7d..34e5ff72ce87 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4703,6 +4703,11 @@ static int mem_cgroup_css_online(struct 
cgroup_subsys_state *css)
 {
        struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 
+       /*
+        * A memcg must be visible for memcg_expand_shrinker_maps()
+        * by the time the maps are allocated. So, we allocate maps
+        * here, when for_each_mem_cgroup() can't skip it.
+        */
        if (memcg_alloc_shrinker_maps(memcg)) {
                mem_cgroup_id_remove(memcg);
                return -ENOMEM;