memcg: add mem_cgroup_cancel_charge()

There are some places calling both res_counter_uncharge() and css_put() to
cancel the charge and the refcnt we have got by mem_cgroup_tyr_charge().

This patch introduces mem_cgroup_cancel_charge() and call it in those
places.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1499,6 +1499,21 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	return -ENOMEM;
 }
 
+/*
+ * Somemtimes we have to undo a charge we got by try_charge().
+ * This function is for that and do uncharge, put css's refcnt.
+ * gotten by try_charge().
+ */
+static void mem_cgroup_cancel_charge(struct mem_cgroup *mem)
+{
+	if (!mem_cgroup_is_root(mem)) {
+		res_counter_uncharge(&mem->res, PAGE_SIZE);
+		if (do_swap_account)
+			res_counter_uncharge(&mem->memsw, PAGE_SIZE);
+	}
+	css_put(&mem->css);
+}
+
 /*
  * A helper function to get mem_cgroup from ID. must be called under
  * rcu_read_lock(). The caller must check css_is_removed() or some if
@@ -1565,12 +1580,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
 	lock_page_cgroup(pc);
 	if (unlikely(PageCgroupUsed(pc))) {
 		unlock_page_cgroup(pc);
-		if (!mem_cgroup_is_root(mem)) {
-			res_counter_uncharge(&mem->res, PAGE_SIZE);
-			if (do_swap_account)
-				res_counter_uncharge(&mem->memsw, PAGE_SIZE);
-		}
-		css_put(&mem->css);
+		mem_cgroup_cancel_charge(mem);
 		return;
 	}
 
@@ -1734,14 +1744,7 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
 cancel:
 	put_page(page);
 uncharge:
-	/* drop extra refcnt by try_charge() */
-	css_put(&parent->css);
-	/* uncharge if move fails */
-	if (!mem_cgroup_is_root(parent)) {
-		res_counter_uncharge(&parent->res, PAGE_SIZE);
-		if (do_swap_account)
-			res_counter_uncharge(&parent->memsw, PAGE_SIZE);
-	}
+	mem_cgroup_cancel_charge(parent);
 	return ret;
 }
 
@@ -1958,12 +1961,7 @@ void mem_cgroup_cancel_charge_swapin(struct mem_cgroup *mem)
 		return;
 	if (!mem)
 		return;
-	if (!mem_cgroup_is_root(mem)) {
-		res_counter_uncharge(&mem->res, PAGE_SIZE);
-		if (do_swap_account)
-			res_counter_uncharge(&mem->memsw, PAGE_SIZE);
-	}
-	css_put(&mem->css);
+	mem_cgroup_cancel_charge(mem);
 }
 
 static void