Date: Tue, 17 Aug 2010 11:41:52 -0500 (CDT)
From: Christoph Lameter <cl@linux-foundation.org>
To: Andi Kleen <andi@firstfloor.org>
cc: Jeff Moyer <jmoyer@redhat.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mel@csn.ul.ie>,
        Wu Fengguang <fengguang.wu@intel.com>,
        "Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>,
        linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [RFC] [PATCH 2/4] dio: add page locking for direct I/O
In-Reply-To: <20100817142144.GB18161@basil.fritz.box>
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References: <1281432464-14833-1-git-send-email-n-horiguchi@ah.jp.nec.com> <alpine.DEB.2.00.1008110806070.673@router.home> <20100812075323.GA6112@spritzera.linux.bs1.fc.nec.co.jp> <20100812075941.GD6112@spritzera.linux.bs1.fc.nec.co.jp>
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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@ah.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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