Re: [PATCH v2 1/7] mm/thp: fix __split_huge_pmd_locked() for migration PMD

Ralph Campbell
2020-09-02T22:02:46Z

On 9/2/20 2:47 PM, Zi Yan wrote:
> On 2 Sep 2020, at 12:58, Ralph Campbell wrote:
>
>> A migrating transparent huge page has to already be unmapped. Otherwise,
>> the page could be modified while it is being copied to a new page and
>> data could be lost. The function __split_huge_pmd() checks for a PMD
>> migration entry before calling __split_huge_pmd_locked() leading one to
>> think that __split_huge_pmd_locked() can handle splitting a migrating PMD.
>> However, the code always increments the page->_mapcount and adjusts the
>> memory control group accounting assuming the page is mapped.
>> Also, if the PMD entry is a migration PMD entry, the call to
>> is_huge_zero_pmd(*pmd) is incorrect because it calls pmd_pfn(pmd) instead
>> of migration_entry_to_pfn(pmd_to_swp_entry(pmd)).
>> Fix these problems by checking for a PMD migration entry.
>>
>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>
> Thanks for the fix. You can add Reviewed-by: Zi Yan <ziy@nvidia.com>
>
> I think you also want to add the Fixes tag and cc stable.
>
> Fixes 84c3fc4e9c56 (“mm: thp: check pmd migration entry in common path”)
> cc: stable@vger.kernel.org # 4.14+

Thanks, I'll add these.