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

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Message-ID: <20100817094007.GA18161@basil.fritz.box>
Date:	Tue, 17 Aug 2010 11:40:08 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.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)

> 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?

-Andi

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