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

Vlastimil Babka
2015-06-22T16:01:48Z

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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