[BUGFIX][PATCH -mm] fix bad call of memcg_oom_recover at cancel move.

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

From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

When cgroup_cancel_attach() is called via cgroup_attach_task(),
mem_cgroup_clear_mc() can be called even when any migration
was done. In such case, mc.to and mc.from is NULL.

But, memcg-clean-up-waiting-move-acct-v2.patch
doesn't handle this correctly and pass NULL to memcg_oom_recover.
fix it.

BUG: unable to handle kernel paging request at 000000000000114c
IP: [<ffffffff81153bb9>] memcg_oom_recover+0x9/0x30
PGD 61ce4b067 PUD 613ea0067 PMD 0
Oops: 0000 [#1] SMP
<snip>
Call Trace:
 [<ffffffff81155359>] mem_cgroup_clear_mc+0x119/0x1c0
 [<ffffffff811554de>] mem_cgroup_cancel_attach+0xe/0x10
 [<ffffffff810b619c>] cgroup_attach_task+0x26c/0x2c0
 [<ffffffff810b6257>] cgroup_tasks_write+0x67/0x1c0
 [<ffffffff81121555>] ? might_fault+0xa5/0xb0
 [<ffffffff8112150c>] ? might_fault+0x5c/0xb0
 [<ffffffff810b40a2>] cgroup_file_write+0x2d2/0x330
 [<ffffffff81093aa2>] ? print_lock_contention_bug+0x22/0xf0
 [<ffffffff81259fef>] ? security_file_permission+0x1f/0x80
 [<ffffffff8115d998>] vfs_write+0xc8/0x190
 [<ffffffff8115e3a1>] sys_write+0x51/0x90
 [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b
Code: 20 48 39 43 20 41 bc f0 ff ff ff 75 c7 45 88 ae 48 11 00 00 45 31 e4 eb bb 66 0f 1f 84 00 00 00 00 00 55 48 89 e5 0f 1f 44 00 00 <8b> 87 4c 11 00 00 85 c0 75 05 c9 c3 0f 1f 00 48 89 f9 31 d2 be
RIP  [<ffffffff81153bb9>] memcg_oom_recover+0x9/0x30

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: mmotm-2.6.35-0611/mm/memcontrol.c
===================================================================
--- mmotm-2.6.35-0611.orig/mm/memcontrol.c
+++ mmotm-2.6.35-0611/mm/memcontrol.c
@@ -4485,8 +4485,10 @@ static void mem_cgroup_clear_mc(void)
 	mc.to = NULL;
 	mc.moving_task = NULL;
 	spin_unlock(&mc.lock);
-	memcg_oom_recover(from);
-	memcg_oom_recover(to);
+	if (from)
+		memcg_oom_recover(from);
+	if (to)
+		memcg_oom_recover(to);
 	wake_up_all(&mc.waitq);
 }
 

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