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

Vlastimil Babka
2015-06-11T09:50:01Z

On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> Original split_huge_page() combined two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> implements split_huge_pmd() which split given PMD without splitting
> other PMDs this page mapped with or underlying compound page.
>
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@oracle.com>

[...]

> +
> + if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> + /* Last compound_mapcount is gone. */
> + __dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
> + if (PageDoubleMap(page)) {
> + /* No need in mapcount reference anymore */
> + ClearPageDoubleMap(page);
> + for (i = 0; i < HPAGE_PMD_NR; i++)
> + atomic_dec(&page[i]._mapcount);
> + }
> + } else if (!TestSetPageDoubleMap(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.
> + */
> + for (i = 0; i < HPAGE_PMD_NR; i++)
> + atomic_inc(&page[i]._mapcount);

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?

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.