Re: [PATCHv6 29/36] thp: implement split_huge_pmd()

[Vlastimil Babka]

On 06/22/2015 01:14 PM, Kirill A. Shutemov wrote:
> On Thu, Jun 11, 2015 at 11:49:48AM +0200, Vlastimil Babka wrote:
> > On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> >
> > The order of actions here means that between TestSetPageDoubleMap() and the
> > atomic incs, anyone calling page_mapcount() on one of the pages not
> > processed by the for loop yet, will see a value lower by 1 from what he
> > should see. I wonder if that can cause any trouble somewhere, especially if
> > there's only one other compound mapping and page_mapcount() will return 0
> > instead of 1?
>
> Good catch. Thanks.
>
> What about this?
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0f1f5731a893..cd0e6addb662 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2636,15 +2636,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                          for (i = 0; i < HPAGE_PMD_NR; i++)
>                                  atomic_dec(&page[i]._mapcount);
>                  }
> -       } else if (!TestSetPageDoubleMap(page)) {
> +       } else if (!PageDoubleMap(page)) {
>                  /*
>                   * The first PMD split for the compound page and we still
>                   * have other PMD mapping of the page: bump _mapcount in
>                   * every small page.
> +                *
>                   * This reference will go away with last compound_mapcount.
> +                *
> +                * Note, we need to increment mapcounts before setting
> +                * PG_double_map to avoid false-negative page_mapped().
>                   */
>                  for (i = 0; i < HPAGE_PMD_NR; i++)
>                          atomic_inc(&page[i]._mapcount);
> +
> +               if (TestSetPageDoubleMap(page)) {
> +                       /* Race with another  __split_huge_pmd() for the page */
> +                       for (i = 0; i < HPAGE_PMD_NR; i++)
> +                               atomic_dec(&page[i]._mapcount);
> +               }
>          }

Yeah that should work.

>          smp_wmb(); /* make pte visible before pmd */



> > Conversely, when clearing PageDoubleMap() above (or in one of those rmap
> > functions IIRC), one could see mapcount inflated by one. But I guess that's
> > less dangerous.
>
> I think it's safe.

OK.

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