Re: [PATCH] mm: Fix possible PMD dirty bit lost in set_pmd_migration_entry()

[Zi Yan]

On 20 Feb 2020, at 2:52, Huang, Ying wrote:

> From: Huang Ying <ying.huang@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  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_mapped_walk *pvmw,
>                 return;
>
>         flush_cache_range(vma, address, address + HPAGE_PMD_SIZE);
> -       pmdval = *pvmw->pmd;
> -       pmdp_invalidate(vma, address, pvmw->pmd);
> +       pmdval = pmdp_invalidate(vma, address, pvmw->pmd);
>         if (pmd_dirty(pmdval))
>                 set_page_dirty(page);
>         entry = make_migration_entry(page, pmd_write(pmdval));
> --
> 2.25.0

Looks good to me. Thanks.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>


â
Best Regards,
Yan Zi

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