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

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Date:	Fri, 21 Dec 2012 13:51:26 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Mel Gorman <mgorman@...e.de>, 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>,
	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 10:21 AM, Hugh Dickins <hughd@...gle.com> wrote:
> On Fri, 21 Dec 2012, Linus Torvalds wrote:
>>
>> compared to the diseased abortion you just posted.
>
> I'm picking up a vibe that you don't entirely like Mel's approach.

Good job. I was a bit nervous that I was being too subtle.

> I don't understand David's and Mel's remarks about the "shared pages"
> check making Sasha's warning unlikely: page_mapcount has nothing to do
> with whether a page belongs to shm/shmem/tmpfs, and it's easy enough
> to reproduce Sasha's warning on the current git tree.  "mount -o
> remount,mpol=local /tmp" or something like that is useful in testing.

I think that Mel and David may talk about the mutex actually blocking
(not just the debug message possibly triggering).

> I wish wish wish I had time to spend on this today, but I don't.
> And I've not looked to see (let alone tested) whether it's easy
> to revert Mel's mutex then add in Kosaki's patch (which I didn't
> look at so have no opinion on).

I don't actually have Kosaki's patch either, just the description of
it. We've done that kind of "preallocate before taking the lock"
before, though.

> Shall we go for Peter/David's mutex+spinlock for rc1 - I assume
> they both tested that - with a promise to do better in rc2?

Well, if the plan is to fix it for rc2, then there is no point in
putting a workaround in now, since actually hitting the problem (as
opposed to seeing the warning) is presumably much harder.

               Linus
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