linux-kernel - Re: [RFC PATCH] take all anon_vma locks in anon_vma

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Message-ID: <4BD879F7.1020102@redhat.com>
Date:	Wed, 28 Apr 2010 14:09:59 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	Mel Gorman <mel@....ul.ie>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] take all anon_vma locks in anon_vma_lock

On 04/28/2010 02:03 PM, Andrea Arcangeli wrote:
> On Wed, Apr 28, 2010 at 01:47:19PM -0400, Rik van Riel wrote:
>>   static inline void anon_vma_unlock(struct vm_area_struct *vma)
>
> never mind as this is RFC, lock is clear enough
>
>> @@ -1762,7 +1760,8 @@ static int expand_downwards(struct vm_area_struct *vma,
>>   	if (error)
>>   		return error;
>>
>> -	anon_vma_lock(vma);
>> +	spin_lock(&mm->page_table_lock);
>> +	anon_vma_lock(vma,&mm->page_table_lock);
>
> This will cause a lock inversion (page_table_lock can only be taken
> after the anon_vma lock). I don't immediately see why the
> page_table_lock here though?

We need to safely walk the vma->anon_vma_chain /
anon_vma_chain->same_vma list.

So much for using the mmap_sem for read + the
page_table_lock to lock the anon_vma_chain list.

We'll need a new lock somewhere, probably in the
mm_struct since one per process seems plenty.

I'll add that in the next version of the patch.

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