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>
—
Best Regards,
Yan Zi
Attachment:
signature.asc
Description: OpenPGP digital signature
