On Thu, Feb 20, 2020 at 03:52:20PM +0800, 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>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
--
Kirill A. Shutemov
