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

[Vlastimil Babka]

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@xxxxxxxxxxxxxxx>
> Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>

[...]

> @@ -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.

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