linux-kernel - Re: [patch for-3.7 v3] mm, mempolicy: hold task->mempolicy refcount while reading numa

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Date:	Fri, 19 Oct 2012 15:15:18 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, bhutchings@...arflare.com,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch for-3.7 v3] mm, mempolicy: hold task->mempolicy refcount
 while reading numa_maps.

On Fri, Oct 19, 2012 at 4:35 AM, Kamezawa Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> (2012/10/19 5:03), David Rientjes wrote:
>>
>> On Thu, 18 Oct 2012, Kamezawa Hiroyuki wrote:
>>>
>>> @@ -132,7 +162,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
>>>         tail_vma = get_gate_vma(priv->task->mm);
>>>         priv->tail_vma = tail_vma;
>>> -
>>> +       hold_task_mempolicy(priv);
>>>         /* Start with last addr hint */
>>>         vma = find_vma(mm, last_addr);
>>>         if (last_addr && vma) {
>>> @@ -159,6 +189,7 @@ out:
>>>         if (vma)
>>>                 return vma;
>>>   +     release_task_mempolicy(priv);
>>>         /* End of vmas has been reached */
>>>         m->version = (tail_vma != NULL)? 0: -1UL;
>>>         up_read(&mm->mmap_sem);
>>
>>
>> Otherwise looks good, but please remove the two task_lock()'s in
>> show_numa_map() that I added as part of this since you're replacing the
>> need for locking.
>>
> Thank you for your review.
> How about this ?
>
> ==
> From c5849c9034abeec3f26bf30dadccd393b0c5c25e Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Date: Fri, 19 Oct 2012 17:00:55 +0900
> Subject: [PATCH] hold task->mempolicy while numa_maps scans.
>
>  /proc/<pid>/numa_maps scans vma and show mempolicy under
>  mmap_sem. It sometimes accesses task->mempolicy which can
>  be freed without mmap_sem and numa_maps can show some
>  garbage while scanning.
>
> This patch tries to take reference count of task->mempolicy at reading
> numa_maps before calling get_vma_policy(). By this, task->mempolicy
> will not be freed until numa_maps reaches its end.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> V2->v3
>  -  updated comments to be more verbose.
>  -  removed task_lock() in numa_maps code.
> V1->V2
>  -  access task->mempolicy only once and remember it.  Becase kernel/exit.c
>     can overwrite it.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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