linux-kernel - Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in numa

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Date:	Thu, 25 Oct 2012 13:23:48 -0400
From:	Sasha Levin <levinsasha928@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	David Rientjes <rientjes@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	bhutchings@...arflare.com,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in
 numa_maps

On 10/25/2012 10:39 AM, Peter Zijlstra wrote:
> On Thu, 2012-10-25 at 14:19 +0200, Peter Zijlstra wrote:
>> On Wed, 2012-10-24 at 17:08 -0700, David Rientjes wrote:
>>> Ok, this looks the same but it's actually a different issue: 
>>> mpol_misplaced(), which now only exists in linux-next and not in 3.7-rc2, 
>>> calls get_vma_policy() which may take the shared policy mutex.  This 
>>> happens while holding page_table_lock from do_huge_pmd_numa_page() but 
>>> also from do_numa_page() while holding a spinlock on the ptl, which is 
>>> coming from the sched/numa branch.
>>>
>>> Is there anyway that we can avoid changing the shared policy mutex back 
>>> into a spinlock (it was converted in b22d127a39dd ["mempolicy: fix a race 
>>> in shared_policy_replace()"])?
>>>
>>> Adding Peter, Rik, and Mel to the cc. 
>>
>> Urgh, crud I totally missed that.
>>
>> So the problem is that we need to compute if the current page is placed
>> 'right' while holding pte_lock in order to avoid multiple pte_lock
>> acquisitions on the 'fast' path.
>>
>> I'll look into this in a bit, but one thing that comes to mind is having
>> both a spnilock and a mutex and require holding both for modification
>> while either one is sufficient for read.
>>
>> That would allow sp_lookup() to use the spinlock, while insert and
>> replace can hold both.
>>
>> Not sure it will work for this, need to stare at this code a little
>> more.
> 
> So I think the below should work, we hold the spinlock over both rb-tree
> modification as sp free, this makes mpol_shared_policy_lookup() which
> returns the policy with an incremented refcount work with just the
> spinlock.
> 
> Comments?
> 
> ---

It made the warnings I've reported go away.


Thanks,
Sasha

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