linux-kernel - Re: [PATCH 0/9] Hugepage migration (v2)

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Date:	Wed, 18 Aug 2010 09:46:23 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	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>
Subject: Re: [PATCH 0/9] Hugepage migration (v2)

On Wed, Aug 18, 2010 at 04:32:34PM +0900, Naoya Horiguchi wrote:
> On Tue, Aug 17, 2010 at 11:40:08AM +0200, Andi Kleen wrote:
> > > When get_user_pages_fast() is called before try_to_unmap(),
> > > direct I/O code increments refcount on the target page.
> > > Because this refcount is not associated to the mapping,
> > > migration code will find remaining refcounts after try_to_unmap()
> > > unmaps all mappings. Then refcount check decides migration to fail,
> > > so direct I/O is continued safely.
> > 
> > This would imply that direct IO can make migration fail arbitarily.
> > Also not good. Should we add some retries, at least for the soft offline
> > case?
> 
> Soft offline is kicked from userspace, so the retry logic can be implemented
> in userspace. However, currently we can't distinguish migration failure from

I don't think user space is the right place for retry logic.
It doesn't really have enough information to make a good decision when
to reply.

Also I would consider requiring user space to work around kernel problems like
that bad design.


-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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