linux-kernel - Re: [RFC] [PATCH 2/4] dio: add page locking for direct I/O

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Message-ID: <alpine.DEB.2.00.1008171140300.11174@router.home>
Date:	Tue, 17 Aug 2010 11:41:52 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Jeff Moyer <jmoyer@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Jun'ichi Nomura <j-nomura@...jp.nec.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC] [PATCH 2/4] dio: add page locking for direct I/O

On Tue, 17 Aug 2010, Andi Kleen wrote:

> On Tue, Aug 17, 2010 at 09:46:56AM -0400, Jeff Moyer wrote:
> > Naoya Horiguchi <n-horiguchi@...jp.nec.com> writes:
> >
> > > BTW, from the discussion with Christoph I noticed my misunderstanding
> > > about the necessity of additional page locking. It would seem that
> > > without page locking there is no danger of racing between direct I/O and
> > > page migration. So I retract this additional locking patch-set.
> >
> > OK, great!  ;-)
>
> Well it sounds like we still may need something. It isn't good if O_DIRECT
> can starve (or DoS) migration.

Anything that increments a page refcount can interfere with migration and
cause migration to be aborted and retried later. Not something new. There
never was any guarantee that migration must succeed.


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