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

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Date:	Fri, 21 Dec 2012 14:02:04 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	David Rientjes <rientjes@...gle.com>,
	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>,
	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 Fri, Dec 21, 2012 at 11:58 AM, Mel Gorman <mgorman@...e.de> wrote:
>
> Kosaki's patch does not fix the actual problem with NUMA hinting
> faults. Converting to a spinlock is nice but we'd still hold the PTL at
> the time sp_alloc is called and potentially allocating GFP_KERNEL with a
> spinlock held.

The problem I saw reported - and the problem that the "mutex+spinlock"
patch was fixing - wasn't actually sp_alloc(), but just sp_lookup()
through mpol_shared_policy_lookup().

And converting that to a spinlock would definitely fix it - taking
that spinlock quickly for the lookup while holding the pt lock is
fine.

Now, if we have to call sp_alloc() too at some point, that's
different, but that wouldn't be helped by the "mutex+spinlock" patch
(that started this thread) anyway.

> At the risk of making your head explode, here is another patch.

So I don't hate this patch, but I don't see the point of your games in
do_pmd_numa_page(). I'm not seeing the allocation in mpol_misplaced(),
and that wasn't what the original report was.

The backtrace you quote is literally *only* about the fact that you
cannot take a mutex inside a spinlock. No allocation, just a lookup.

So where's the sp_alloc()?

                 Linus
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