Date: Thu, 25 Oct 2012 13:22:00 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
To: Peter Zijlstra <peterz@infradead.org>
cc: Sasha Levin <levinsasha928@gmail.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>, Dave Jones <davej@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>, bhutchings@solarflare.com,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Hugh Dickins <hughd@google.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in
 numa_maps
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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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