On Mon, Jun 29, 2020 at 04:19:53PM +0100, Matthew Wilcox wrote:
> This removes a few instructions from functions which need to know how many
> pages are in a compound page. The storage used is either page->mapping
> on 64-bit or page->index on 32-bit. Both of these are fine to overlay
> on tail pages.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> include/linux/mm.h | 5 ++++-
> include/linux/mm_types.h | 1 +
> mm/page_alloc.c | 5 +++--
> 3 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dc7b87310c10..af0305ad090f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -911,12 +911,15 @@ static inline int compound_pincount(struct page *page)
> static inline void set_compound_order(struct page *page, unsigned int order)
> {
> page[1].compound_order = order;
> + page[1].compound_nr = 1U << order;
^^^
1UL?
Ira
> }
>
> /* Returns the number of pages in this potentially compound page. */
> static inline unsigned long compound_nr(struct page *page)
> {
> - return 1UL << compound_order(page);
> + if (!PageHead(page))
> + return 1;
> + return page[1].compound_nr;
> }
>
> /* Returns the number of bytes in this potentially compound page. */
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 64ede5f150dc..561ed987ab44 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -134,6 +134,7 @@ struct page {
> unsigned char compound_dtor;
> unsigned char compound_order;
> atomic_t compound_mapcount;
> + unsigned int compound_nr; /* 1 << compound_order */
> };
> struct { /* Second tail page of compound page */
> unsigned long _compound_pad_1; /* compound_head */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48eb0f1410d4..c7beb5f13193 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -673,8 +673,6 @@ void prep_compound_page(struct page *page, unsigned int order)
> int i;
> int nr_pages = 1 << order;
>
> - set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
> - set_compound_order(page, order);
> __SetPageHead(page);
> for (i = 1; i < nr_pages; i++) {
> struct page *p = page + i;
> @@ -682,6 +680,9 @@ void prep_compound_page(struct page *page, unsigned int order)
> p->mapping = TAIL_MAPPING;
> set_compound_head(p, page);
> }
> +
> + set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
> + set_compound_order(page, order);
> atomic_set(compound_mapcount_ptr(page), -1);
> if (hpage_pincount_available(page))
> atomic_set(compound_pincount_ptr(page), 0);
> --
> 2.27.0
>
>
