memcg: check for proper lock held in mem_cgroup_update_page_stat

We should call mem_cgroup_begin_update_page_stat() before
mem_cgroup_update_page_stat() to get proper locks, however the latter
doesn't do any checking that we use proper locking, which would be hard.
Suggested by Michal Hock we could at least test for rcu_read_lock_held()
because RCU is held if !mem_cgroup_disabled().

Signed-off-by: Sha Zhengju <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Reviewed-by: Greg Thelen <[e-mail]>
Cc: Fengguang Wu <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 	if (mem_cgroup_disabled())
 		return;
 
+	VM_BUG_ON(!rcu_read_lock_held());
 	memcg = pc->mem_cgroup;
 	if (unlikely(!memcg || !PageCgroupUsed(pc)))
 		return;