memcg: avoid refill_stock for root memcg

We never charge the page counters of root memcg, so there is no need to
put root memcg in the memcg stock.  At the moment, refill_stock() can be
called from try_charge_memcg(), obj_cgroup_uncharge_pages() and
mem_cgroup_uncharge_skmem().

The try_charge_memcg() and mem_cgroup_uncharge_skmem() are never called
with root memcg, so those are fine.  However obj_cgroup_uncharge_pages()
can potentially call refill_stock() with root memcg if the objcg object
has been reparented over to the root memcg.  Let's just avoid
refill_stock() from obj_cgroup_uncharge_pages() for root memcg.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shakeel Butt <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/memcontrol.c

@@ -2643,7 +2643,8 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 
 	mod_memcg_state(memcg, MEMCG_KMEM, -nr_pages);
 	memcg1_account_kmem(memcg, -nr_pages);
-	refill_stock(memcg, nr_pages);
+	if (!mem_cgroup_is_root(memcg))
+		refill_stock(memcg, nr_pages);
 
 	css_put(&memcg->css);
 }