linux-kernel - Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside spinlock when reading numa

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Message-ID: <CAHGf_=r9ynudaetANyEng64OSXOsLqdQ1SXHb0Z9AnAr23ahZw@mail.gmail.com>
Date:	Fri, 19 Oct 2012 02:51:29 -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 v2] mm, mempolicy: avoid taking mutex inside
 spinlock when reading numa_maps

>> Can't we have another way to fix ? like this ? too ugly ?
>> Again, I'm sorry if I misunderstand the points.
>>
> Sorry this patch itself may be buggy. please don't test..
> I missed that kernel/exit.c sets task->mempolicy to be NULL.
> fixed one here.
>
> --
> From 5581c71e68a7f50e52fd67cca00148911023f9f5 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Date: Thu, 18 Oct 2012 13:50:29 +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>
>
> 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>

Ok, this is acceptable to me. go ahead.
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