memcg: don't need to free memcg via RCU or workqueue

Now memcg has the same life cycle with its corresponding cgroup, and a
cgroup is freed via RCU and then mem_cgroup_css_free() will be called in
a work function, so we can simply call __mem_cgroup_free() in
mem_cgroup_css_free().

This actually reverts commit 59927fb984d ("memcg: free mem_cgroup by RCU
to fix oops").

Signed-off-by: Li Zefan <lizefan@huawei.com>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Glauber Costa <glommer@openvz.org>
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

@@ -263,28 +263,10 @@ struct mem_cgroup {
 	/* vmpressure notifications */
 	struct vmpressure vmpressure;
 
-	union {
-		/*
-		 * the counter to account for mem+swap usage.
-		 */
-		struct res_counter memsw;
-
-		/*
-		 * rcu_freeing is used only when freeing struct mem_cgroup,
-		 * so put it into a union to avoid wasting more memory.
-		 * It must be disjoint from the css field.  It could be
-		 * in a union with the res field, but res plays a much
-		 * larger part in mem_cgroup life than memsw, and might
-		 * be of interest, even at time of free, when debugging.
-		 * So share rcu_head with the less interesting memsw.
-		 */
-		struct rcu_head rcu_freeing;
-		/*
-		 * We also need some space for a worker in deferred freeing.
-		 * By the time we call it, rcu_freeing is no longer in use.
-		 */
-		struct work_struct work_freeing;
-	};
+	/*
+	 * the counter to account for mem+swap usage.
+	 */
+	struct res_counter memsw;
 
 	/*
 	 * the counter to account for kernel memory usage.
@@ -6211,29 +6193,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 		vfree(memcg);
 }
 
-
-/*
- * Helpers for freeing a kmalloc()ed/vzalloc()ed mem_cgroup by RCU,
- * but in process context.  The work_freeing structure is overlaid
- * on the rcu_freeing structure, which itself is overlaid on memsw.
- */
-static void free_work(struct work_struct *work)
-{
-	struct mem_cgroup *memcg;
-
-	memcg = container_of(work, struct mem_cgroup, work_freeing);
-	__mem_cgroup_free(memcg);
-}
-
-static void free_rcu(struct rcu_head *rcu_head)
-{
-	struct mem_cgroup *memcg;
-
-	memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
-	INIT_WORK(&memcg->work_freeing, free_work);
-	schedule_work(&memcg->work_freeing);
-}
-
 /*
  * Returns the parent mem_cgroup in memcgroup hierarchy with hierarchy enabled.
  */
@@ -6383,7 +6342,7 @@ static void mem_cgroup_css_free(struct cgroup *cont)
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
 
 	memcg_destroy_kmem(memcg);
-	call_rcu(&memcg->rcu_freeing, free_rcu);
+	__mem_cgroup_free(memcg);
 }
 
 #ifdef CONFIG_MMU