Re: [PATCHv6 27/36] mm: differentiate page_mapped() from page_mapcount() for compound pages

Vlastimil Babka
2015-06-10T14:34:35Z

On 06/03/2015 07:05 PM, Kirill A. Shutemov wrote:
> Let's define page_mapped() to be true for compound pages if any
> sub-pages of the compound page is mapped (with PMD or PTE).
>
> On other hand page_mapcount() return mapcount for this particular small
> page.
>
> This will make cases like page_get_anon_vma() behave correctly once we
> allow huge pages to be mapped with PTE.
>
> Most users outside core-mm should use page_mapcount() instead of
> page_mapped().
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@oracle.com>

[...]

> @@ -917,10 +917,21 @@ static inline pgoff_t page_file_index(struct page *page)
>
> /*
> * Return true if this page is mapped into pagetables.
> + * For compound page it returns true if any subpage of compound page is mapped.
> */
> -static inline int page_mapped(struct page *page)
> +static inline bool page_mapped(struct page *page)
> {
> - return atomic_read(&(page)->_mapcount) + compound_mapcount(page) >= 0;
> + int i;
> + if (likely(!PageCompound(page)))
> + return atomic_read(&page->_mapcount) >= 0;
> + page = compound_head(page);
> + if (compound_mapcount(page))

Same optimization opportunity as I pointed out in previous patch for
page_mapcount().

> + return true;
> + for (i = 0; i < hpage_nr_pages(page); i++) {
> + if (atomic_read(&page[i]._mapcount) >= 0)
> + return true;
> + }
> + return true;
> }
>
> /*
> diff --git a/mm/filemap.c b/mm/filemap.c
> index cb41cf3069d2..c6cf03303ded 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -202,7 +202,7 @@ void __delete_from_page_cache(struct page *page, void *shadow)
> __dec_zone_page_state(page, NR_FILE_PAGES);
> if (PageSwapBacked(page))
> __dec_zone_page_state(page, NR_SHMEM);
> - BUG_ON(page_mapped(page));
> + VM_BUG_ON_PAGE(page_mapped(page), page);
>
> /*
> * At this point page must be either written or cleaned by truncate.
>