[PATCH] memcg: fix oops in mem_cgroup_shrink_usage

[Hugh Dickins]

Got an oops in mem_cgroup_shrink_usage() when testing loop over tmpfs:
yes, of course, loop0 has no mm: other entry points check but this didn't.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---

 mm/memcontrol.c |    2 ++
 1 file changed, 2 insertions(+)

--- 2.6.27-rc2/mm/memcontrol.c	2008-08-06 08:36:20.000000000 +0100
+++ linux/mm/memcontrol.c	2008-08-06 09:05:26.000000000 +0100
@@ -796,6 +796,8 @@ int mem_cgroup_shrink_usage(struct mm_st
 
 	if (mem_cgroup_subsys.disabled)
 		return 0;
+	if (!mm)
+		return 0;
 
 	rcu_read_lock();
 	mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
--
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