linux-kernel - Re: [PATCH 3/3] mm: migrate: Stabilise page count when migrating transparent hugepages

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Message-ID: <20170608104702.rqrczxh2wcyfril7@black.fi.intel.com>
Date:   Thu, 8 Jun 2017 13:47:03 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, akpm@...ux-foundation.org,
        Punit.Agrawal@....com, mgorman@...e.de, steve.capper@....com
Subject: Re: [PATCH 3/3] mm: migrate: Stabilise page count when migrating
 transparent hugepages

On Tue, Jun 06, 2017 at 06:58:36PM +0100, Will Deacon wrote:
> When migrating a transparent hugepage, migrate_misplaced_transhuge_page
> guards itself against a concurrent fastgup of the page by checking that
> the page count is equal to 2 before and after installing the new pmd.
> 
> If the page count changes, then the pmd is reverted back to the original
> entry, however there is a small window where the new (possibly writable)
> pmd is installed and the underlying page could be written by userspace.
> Restoring the old pmd could therefore result in loss of data.
> 
> This patch fixes the problem by freezing the page count whilst updating
> the page tables, which protects against a concurrent fastgup without the
> need to restore the old pmd in the failure case (since the page count can
> no longer change under our feet).
> 
> Cc: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Will Deacon <will.deacon@....com>

Looks correct to me.

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov