Subject: memcg fix prepare migration

From: Andrea Arcangeli <aarcange@redhat.com>

If a signal is pending (task being killed by sigkill) __mem_cgroup_try_charge
will write NULL into &mem, and css_put will oops on null pointer dereference.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffff810fc6cc>] mem_cgroup_prepare_migration+0x7c/0xc0
PGD a5d89067 PUD a5d8a067 PMD 0
Oops: 0000 [#1] SMP
last sysfs file: /sys/devices/platform/microcode/firmware/microcode/loading
CPU 0
Modules linked in: nfs lockd nfs_acl auth_rpcgss sunrpc acpi_cpufreq pcspkr sg [last unloaded: microcode]

Pid: 5299, comm: largepages Tainted: G        W  2.6.34-rc3 #3 Penryn1600SLI-110dB/To Be Filled By O.E.M.
RIP: 0010:[<ffffffff810fc6cc>]  [<ffffffff810fc6cc>] mem_cgroup_prepare_migration+0x7c/0xc0

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2444,12 +2445,12 @@ int mem_cgroup_prepare_migration(struct 
 	}
 	unlock_page_cgroup(pc);
 
+	*ptr = mem;
 	if (mem) {
-		ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, &mem, false,
+		ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, ptr, false,
 					      PAGE_SIZE);
 		css_put(&mem->css);
 	}
-	*ptr = mem;
 	return ret;
 }