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Date:	Fri, 10 Jun 2011 14:22:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Xiaotian Feng <xtfeng@...il.com>
Cc:	Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

On Fri, 10 Jun 2011 13:21:46 +0800
Xiaotian Feng <xtfeng@...il.com> wrote:

> On Fri, Jun 10, 2011 at 12:30 PM, KAMEZAWA Hiroyuki <
> kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> >
> > I think this can be a fix.
> > maybe good to CC Oleg.
> 
> ==
> > From dff52fb35af0cf36486965d19ee79e04b59f1dc4 Mon Sep 17 00:00:00 2001
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Date: Fri, 10 Jun 2011 13:15:14 +0900
> > Subject: [PATCH] [BUGFIX] update mm->owner even if no next owner.
> >
> > A panic is reported.
> >
> > > Call Trace:
> > >  [<ffffffff81139792>] mem_cgroup_from_task+0x15/0x17
> > >  [<ffffffff8113a75a>] __mem_cgroup_try_charge+0x148/0x4b4
> > >  [<ffffffff810493f3>] ? need_resched+0x23/0x2d
> > >  [<ffffffff814cbf43>] ? preempt_schedule+0x46/0x4f
> > >  [<ffffffff8113afe8>] mem_cgroup_charge_common+0x9a/0xce
> > >  [<ffffffff8113b6d1>] mem_cgroup_newpage_charge+0x5d/0x5f
> > >  [<ffffffff81134024>] khugepaged+0x5da/0xfaf
> > >  [<ffffffff81078ea0>] ? __init_waitqueue_head+0x4b/0x4b
> > >  [<ffffffff81133a4a>] ? add_mm_counter.constprop.5+0x13/0x13
> > >  [<ffffffff81078625>] kthread+0xa8/0xb0
> > >  [<ffffffff814d13e8>] ? sub_preempt_count+0xa1/0xb4
> > >  [<ffffffff814d5664>] kernel_thread_helper+0x4/0x10
> > >  [<ffffffff814ce858>] ? retint_restore_args+0x13/0x13
> > >  [<ffffffff8107857d>] ? __init_kthread_worker+0x5a/0x5a
> >
> > The code is.
> > >         return container_of(task_subsys_state(p, mem_cgroup_subsys_id),
> > >                                 struct mem_cgroup, css);
> >
> >
> > What happens here is accssing a freed task struct "p" from mm->owner.
> > So, it's doubtful that mm->owner points to freed task struct.
> >
> >
> But from the bug itself, it looks more likely kernel is hitting a freed
> p->cgroups, right?
> If p is already freed, the kernel will fault on
> 781cc62d: 8b 82 fc 08 00 00       mov    0x8fc(%edx),%eax
> 
> Then you will not get a value of 6b6b6b87, right?


%edx here is a pointer for task struct.
Then, task->cgroup == 0x6b6b6b6b. It means "task" is freed.

Thanks,
-Kame




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