memcg: add interface to reset limits

We don't have an interface to reset mem.limit or memsw.limit now.

This patch allows to reset mem.limit or memsw.limit when they are being
set to -1.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/res_counter.c b/kernel/res_counter.c
index bf8e753..e1338f0 100644
--- a/kernel/res_counter.c
+++ b/kernel/res_counter.c
@@ -18,7 +18,7 @@
 void res_counter_init(struct res_counter *counter, struct res_counter *parent)
 {
 	spin_lock_init(&counter->lock);
-	counter->limit = (unsigned long long)LLONG_MAX;
+	counter->limit = RESOURCE_MAX;
 	counter->parent = parent;
 }
 
@@ -133,6 +133,16 @@
 					unsigned long long *res)
 {
 	char *end;
+
+	/* return RESOURCE_MAX(unlimited) if "-1" is specified */
+	if (*buf == '-') {
+		*res = simple_strtoull(buf + 1, &end, 10);
+		if (*res != 1 || *end != '\0')
+			return -EINVAL;
+		*res = RESOURCE_MAX;
+		return 0;
+	}
+
 	/* FIXME - make memparse() take const char* args */
 	*res = memparse((char *)buf, &end);
 	if (*end != '\0')