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Date: Tue, 17 Aug 2010 11:40:08 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux-foundation.org>,
        Andi Kleen <andi@firstfloor.org>,
        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>
Subject: Re: [PATCH 0/9] Hugepage migration (v2)
Message-ID: <20100817094007.GA18161@basil.fritz.box>
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> 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@linux.intel.com -- Speaking for myself only.
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