linux-kernel - Re: [PATCH 14/15] mm: fix TLB flush race between migration, and change_protection

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Message-ID: <00000142c99cf5b0-69cc9987-aa36-4889-af6a-1a45032d0d13-000000@email.amazonses.com>
Date:	Fri, 6 Dec 2013 20:32:56 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Rik van Riel <riel@...hat.com>
cc:	Mel Gorman <mgorman@...e.de>, Alex Thorlton <athorlton@....com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/15] mm: fix TLB flush race between migration, and
 change_protection_range

On Fri, 6 Dec 2013, Rik van Riel wrote:
>
> The basic race looks like this:
>
> CPU A			CPU B			CPU C
>
> 						load TLB entry
> make entry PTE/PMD_NUMA
> 			fault on entry
> 						read/write old page
> 			start migrating page

When you start migrating a page a special page migration entry is
created that will trap all accesses to the page. You can safely flush when
the migration entry is there. Only allow a new PTE/PMD to be put there
*after* the tlb flush.

> 			change PTE/PMD to new page

Dont do that. We have migration entries for a reason.

> 						read/write old page [*]

Should cause a page fault which should put the process to sleep. Process
will safely read the page after the migration entry is removed.

> flush TLB

Establish the new PTE/PMD after the flush removing the migration pte
entry and thereby avoiding the race.

> 						reload TLB from new entry
> 						read/write new page
> 						lose data
>
> [*] the old page may belong to a new user at this point!
>
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