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From:   Zi Yan <ziy@nvidia.com>
To:     "Huang, Ying" <ying.huang@intel.com>
CC:     Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm: Fix possible PMD dirty bit lost in
 set_pmd_migration_entry()
Date:   Thu, 20 Feb 2020 08:18:49 -0500
Message-ID: <51A5D10D-0ABA-4FA2-A749-EBF60DB4FEE6@nvidia.com>
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On 20 Feb 2020, at 2:52, Huang, Ying wrote:

> From: Huang Ying <ying.huang@intel.com>
>
> In set_pmd_migration_entry(), pmdp_invalidate() is used to change PMD
> atomically.  But the PMD is read before that with an ordinary memory
> reading.  If the THP (transparent huge page) is written between the
> PMD reading and pmdp_invalidate(), the PMD dirty bit may be lost, and
> cause data corruption.  The race window is quite small, but still
> possible in theory, so need to be fixed.
>
> The race is fixed via using the return value of pmdp_invalidate() to
> get the original content of PMD, which is a read/modify/write atomic
> operation.  So no THP writing can occur in between.
>
> The race has been introduced when the THP migration support is added
> in the commit 616b8371539a ("mm: thp: enable thp migration in generic
> path").  But this fix depends on the commit d52605d7cb30 ("mm: do not
> lose dirty and accessed bits in pmdp_invalidate()").  So it's easy to
> be backported after v4.16.  But the race window is really small, so it
> may be fine not to backport the fix at all.
>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/huge_memory.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 580098e115bd..b1e069e68189 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3060,8 +3060,7 @@ void set_pmd_migration_entry(struct page_vma_mapp=
ed_walk *pvmw,
>                 return;
>
>         flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
> -       pmdval =3D *pvmw->pmd;
> -       pmdp_invalidate(vma, address, pvmw->pmd);
> +       pmdval =3D pmdp_invalidate(vma, address, pvmw->pmd);
>         if (pmd_dirty(pmdval))
>                 set_page_dirty(page);
>         entry =3D make_migration_entry(page, pmd_write(pmdval));
> --
> 2.25.0

Looks good to me. Thanks.

Reviewed-by: Zi Yan <ziy@nvidia.com>


=E2=80=94
Best Regards,
Yan Zi

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