memcg: fix error path of mem_cgroup_move_parent

There is a bug in error path of mem_cgroup_move_parent.

Extra refcnt got from try_charge should be dropped, and usages incremented
by try_charge should be decremented in both error paths:

    A: failure at get_page_unless_zero
    B: failure at isolate_lru_page

This bug makes this parent directory unremovable.

In case of A, rmdir doesn't return, because res.usage doesn't go down to 0
at mem_cgroup_force_empty even after all the pc in lru are removed.

In case of B, rmdir fails and returns -EBUSY, because it has extra ref
counts even after res.usage goes down to 0.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -994,14 +994,15 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
 	if (pc->mem_cgroup != from)
 		goto out;
 
-	css_put(&from->css);
 	res_counter_uncharge(&from->res, PAGE_SIZE);
 	mem_cgroup_charge_statistics(from, pc, false);
 	if (do_swap_account)
 		res_counter_uncharge(&from->memsw, PAGE_SIZE);
+	css_put(&from->css);
+
+	css_get(&to->css);
 	pc->mem_cgroup = to;
 	mem_cgroup_charge_statistics(to, pc, true);
-	css_get(&to->css);
 	ret = 0;
 out:
 	unlock_page_cgroup(pc);
@@ -1034,8 +1035,10 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
 	if (ret || !parent)
 		return ret;
 
-	if (!get_page_unless_zero(page))
-		return -EBUSY;
+	if (!get_page_unless_zero(page)) {
+		ret = -EBUSY;
+		goto uncharge;
+	}
 
 	ret = isolate_lru_page(page);
 
@@ -1044,19 +1047,23 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
 
 	ret = mem_cgroup_move_account(pc, child, parent);
 
-	/* drop extra refcnt by try_charge() (move_account increment one) */
-	css_put(&parent->css);
 	putback_lru_page(page);
 	if (!ret) {
 		put_page(page);
+		/* drop extra refcnt by try_charge() */
+		css_put(&parent->css);
 		return 0;
 	}
-	/* uncharge if move fails */
+
 cancel:
+	put_page(page);
+uncharge:
+	/* drop extra refcnt by try_charge() */
+	css_put(&parent->css);
+	/* uncharge if move fails */
 	res_counter_uncharge(&parent->res, PAGE_SIZE);
 	if (do_swap_account)
 		res_counter_uncharge(&parent->memsw, PAGE_SIZE);
-	put_page(page);
 	return ret;
 }