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

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Message-ID: <00000142ca7218fe-a5566a24-0ef5-4545-8a98-d33116b7d703-000000@email.amazonses.com>
Date:	Sat, 7 Dec 2013 00:25:44 +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:

> > 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.
>
> A PROT_NONE or NUMA pte is just as effective as a migration pte.
> The only problem is, the TLB flush was not always done...

Ok then what are you trying to fix?

> > Dont do that. We have migration entries for a reason.
>
> We do not have migration entries for hugepages, do we?

Dunno.

> >
> > 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.
>
> That is what this patch does.

If that is the case then this patch would not be needed and the tracking
of state in the mm_struct would not be necessary.
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