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

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Date:	Wed, 18 Aug 2010 16:32:34 +0900
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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 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
"unknown page" error by return value (-EIO in both case for now.)
How about changing return value to -EAGAIN when migration failed?

Thanks,
Naoya Horiguchi
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