linux-kernel - Re: [patch] mm, mempolicy: Introduce spinlock to read shared policy tree

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Date:	Thu, 20 Dec 2012 14:55:22 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Sasha Levin <levinsasha928@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [patch] mm, mempolicy: Introduce spinlock to read shared policy
 tree

On Thu, 20 Dec 2012, Linus Torvalds wrote:

> Going through some old emails before -rc1 rlease..
> 
> What is the status of this patch? The patch that is reported to cause
> the problem hasn't been merged, but that mpol_misplaced() thing did
> happen in commit 771fb4d806a9. And it looks like it's called from
> numa_migrate_prep() under the pte map lock. Or am I missing something?

Andrew pinged both Ingo and I about it privately two weeks ago.  It 
probably doesn't trigger right now because there's no pte_mknuma() on 
shared pages (yet) but will eventually be needed for correctness.  So it's 
not required for -rc1 as it sits in the tree today but will be needed 
later (and hopefully not forgotten about until Sasha fuzzes again).

> See commit 9532fec118d ("mm: numa: Migrate pages handled during a
> pmd_numa hinting fault").
> 
> Am I missing something? Mel, please take another look.
> 
> I despise these kinds of dual-locking models, and am wondering if we
> can't have *just* the spinlock?
> 

Adding KOSAKI to the cc.

This is probably worth discussing now to see if we can't revert 
b22d127a39dd ("mempolicy: fix a race in shared_policy_replace()"), keep it 
only as a spinlock as you suggest, and do what KOSAKI suggested in 
http://marc.info/?l=linux-kernel&m=133940650731255 instead.  I don't think 
it's worth trying to optimize this path at the cost of having both a 
spinlock and mutex.
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