[BUGFIX][PATCH] memcg: fix wrong pointer initialization at page migration when memcg is disabled.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

Lee Schermerhorn reported that he saw bad pointer dereference
in mem_cgroup_end_migration() when he disabled memcg by boot option.

memcg's page migration logic works as

	mem_cgroup_prepare_migration(page, &ptr);
	do page migration
	mem_cgroup_end_migration(page, ptr);

Now, ptr is not initialized in prepare_migration when memcg is disabled
by boot option. This causes panic in end_migration. This patch fixes it.

Reported-by: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux-2.6.32-rc5/mm/memcontrol.c
===================================================================
--- linux-2.6.32-rc5.orig/mm/memcontrol.c
+++ linux-2.6.32-rc5/mm/memcontrol.c
@@ -1990,7 +1990,8 @@ int mem_cgroup_prepare_migration(struct 
 	struct page_cgroup *pc;
 	struct mem_cgroup *mem = NULL;
 	int ret = 0;
-
+	/* this pointer will be checked at end_migration */
+	*ptr = NULL;
 	if (mem_cgroup_disabled())
 		return 0;
 

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