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

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Date:	Thu, 25 Oct 2012 13:22:00 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Sasha Levin <levinsasha928@...il.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 Thu, 25 Oct 2012, Peter Zijlstra wrote:

> 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's rather unfortunate that we need to protect modification with a 
spinlock and a mutex but since sharing was removed in commit 869833f2c5c6 
("mempolicy: remove mempolicy sharing") it requires that sp_alloc() is 
blockable to do the whole mpol_new() and rebind if necessary, which could 
require mm->mmap_sem; it's not as simple as just converting all the 
allocations to GFP_ATOMIC.

It looks as though there is no other alternative other than protecting 
modification with both the spinlock and mutex, which is a clever 
solution, so it looks good to me, thanks!
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