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

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Date:	Wed, 12 Nov 2008 00:25:13 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
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, Nov 12, 2008 at 12:17:22AM +0100, Andrea Arcangeli wrote:
> We don't have to check the page_count vs mapcount later in
> replace_page because we know if anybody started an O_DIRECT read from
> disk, it would have triggered a cow, and the pte_same check that we
> have to do for other reasons would take care of bailing out the
> replace_page.

Ah, for completeness: above I didn't mention the case of O_DIRECT
writes to disk, because we never need to care about those. They're
never a problem. If the page is replaced and the cpu writes to the
page and by doing so triggers a cow that lead to the CPU write to go
in a different page (not the one under dma) it'll be like if the write
to disk completed before the cpu overwritten the page, so result is
undefined. I don't think we've to define the case of somebody doing a
direct read from a location where there's still an o_direct write in
flight either.
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