memcg: add null check to page_cgroup_zoneinfo()

If CONFIG_CGROUP_MEM_RES_CTLR_SWAP=y, page_cgroup::mem_cgroup can be NULL.
Therefore null checking is better.

A later patch uses this function.

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9b4993ee46b315187600f48cac8236cebd7c46af..457d671029b81cf92867955e4ab9edd53aef6e49 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -231,6 +231,9 @@ page_cgroup_zoneinfo(struct page_cgroup *pc)
        int nid = page_cgroup_nid(pc);
        int zid = page_cgroup_zid(pc);
 
+       if (!mem)
+               return NULL;
+
        return mem_cgroup_zoneinfo(mem, nid, zid);
 }