memcg: fold mem_cgroup_stolen

It is only used in __mem_cgroup_begin_update_page_stat(), the name is
confusing and 2 routines for one thing also confuse people, so fold this
function seems more clear.

[akpm@linux-foundation.org: fix typo, per Michal]
Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1594,23 +1594,12 @@ static void mem_cgroup_end_move(struct mem_cgroup *memcg)
 }
 
 /*
- * 2 routines for checking "mem" is under move_account() or not.
+ * A routine for checking "mem" is under move_account() or not.
  *
- * mem_cgroup_stolen() -  checking whether a cgroup is mc.from or not. This
- *			  is used for avoiding races in accounting.  If true,
- *			  pc->mem_cgroup may be overwritten.
- *
- * mem_cgroup_under_move() - checking a cgroup is mc.from or mc.to or
- *			  under hierarchy of moving cgroups. This is for
- *			  waiting at hith-memory prressure caused by "move".
+ * Checking a cgroup is mc.from or mc.to or under hierarchy of
+ * moving cgroups. This is for waiting at high-memory pressure
+ * caused by "move".
  */
-
-static bool mem_cgroup_stolen(struct mem_cgroup *memcg)
-{
-	VM_BUG_ON(!rcu_read_lock_held());
-	return atomic_read(&memcg->moving_account) > 0;
-}
-
 static bool mem_cgroup_under_move(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *from;
@@ -1653,7 +1642,6 @@ static bool mem_cgroup_wait_acct_move(struct mem_cgroup *memcg)
  * Take this lock when
  * - a code tries to modify page's memcg while it's USED.
  * - a code tries to modify page state accounting in a memcg.
- * see mem_cgroup_stolen(), too.
  */
 static void move_lock_mem_cgroup(struct mem_cgroup *memcg,
 				  unsigned long *flags)
@@ -2326,9 +2314,10 @@ again:
 	 * If this memory cgroup is not under account moving, we don't
 	 * need to take move_lock_mem_cgroup(). Because we already hold
 	 * rcu_read_lock(), any calls to move_account will be delayed until
-	 * rcu_read_unlock() if mem_cgroup_stolen() == true.
+	 * rcu_read_unlock().
 	 */
-	if (!mem_cgroup_stolen(memcg))
+	VM_BUG_ON(!rcu_read_lock_held());
+	if (atomic_read(&memcg->moving_account) <= 0)
 		return;
 
 	move_lock_mem_cgroup(memcg, flags);