linux-kernel - Re: [PATCH 0/6] mempolicy memory corruption fixlet

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Date:	Wed, 06 Jun 2012 15:10:53 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>, david@...g.hm,
	Christoph Lameter <cl@...ux.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...gle.com>,
	Dave Jones <davej@...hat.com>, Mel Gorman <mgorman@...e.de>,
	stable@...r.kernel.org, hughd@...gle.com, sivanich@....com
Subject: Re: [PATCH 0/6] mempolicy memory corruption fixlet

(6/5/12 3:17 PM), Andrew Morton wrote:
> On Tue, 5 Jun 2012 12:02:25 -0700
> Linus Torvalds<torvalds@...ux-foundation.org>  wrote:
>
>> I'm coming back to this email thread, because I didn't apply the
>> series due to all the ongoing discussion and hoping that somebody
>> would put changelog fixes and ack notices etc together.
>>
>> I'd also really like to know that the people who saw the problem that
>> caused the current single patch (that this series reverts) would test
>> the whole series. Maybe that happened and I didn't notice it in the
>> threads, but I don't think so.

I'm not surprised this. If many people are interesting to review this area,
mempolicy wouldn't have break so a lot.

>> In fact, right now I'm assuming that the series will eventually come
>> to me through Andrew. Andrew, correct?
>
> yup.
>
> I expect there will be a v2 series (at least).  It's unclear what
> we'll be doing with [2/6]: whether the patch will be reworked, or
> whether Andi misunderstood its effects?

Maybe because Andi didn't join bug fix works in this area for several years?

Currently, mbind(2) is completely broken. A primary role of mbind(2) is to
update memory policy of some vmas and Mel's fix remvoed it. Then, mbind is
almostly no-op. it's a regression.

I'm not clear which point you seems unclear. So, let's repeat a description of
[2/6].

There are two problem now, alloc_pages_vma() has strong and wrong assumption.
vma->policy never have MPOL_F_SHARED and shared_policy->policy must have it.
And, cpusets rebinding ignore mpol->refcnt and updates it forcibly.

The final point is to implement cow. But for it, we need rewrite mpol->rebind
family completely. It doesn't fit for 3.5 timeframe.

The downside of patch [2/6] is very small. because,

A memplicy is only shared three cases, 1) mbind() updates multiple
vmas 2) mbind() updates shmem vma 3) A shared policy splits into two regions
by a part region update.

All of them are rare. Because nobody hit kernel panic until now. Then I don't
think my patch increase memory footprint.

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