linux-kernel - Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another

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Message-ID: <Pine.LNX.4.64.0811112109390.10501@quilx.com>
Date:	Tue, 11 Nov 2008 21:10:45 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kvm@...r.kernel.org, chrisw@...hat.com,
	avi@...hat.com, izike@...ranet.com
Subject: Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from
 one page into another

On Wed, 12 Nov 2008, Andrea Arcangeli wrote:

> So are you checking if there's an unresolved reference only in the
> very place I just quoted in the previous email? If answer is yes: what
> should prevent get_user_pages from running in parallel from another
> thread? get_user_pages will trigger a minor fault and get the elevated
> reference just after you read page_count. To you it looks like there
> is no o_direct in progress when you proceed to the core of migration
> code, but in effect o_direct just started a moment after you read the
> page count.

get_user_pages() cannot get to it since the pagetables have already been
modified. If get_user_pages runs then the fault handling will occur
which will block the thread until migration is complete.
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