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

[Ralph Campbell]

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@xxxxxxxxxx>
>
> Thanks for the fix. You can add Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx # 4.14+

Thanks, I'll add these.