Re: [PATCH 1/7] mm: Store compound_nr as well as compound_order

[Ira Weiny <ira.weiny@intel.com>]

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@infradead.org>
> ---
>  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
> 
>