Re: [PATCH 1/5] mm: Constify a lot of struct page arguments

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Apr 08, 2020 at 08:01:44AM -0700, Matthew Wilcox wrote:
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 4aba6c0c2ba8..a8c3fa076f43 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -221,15 +221,8 @@ struct page {
>  #endif
>  } _struct_page_alignment;
>  
> -static inline atomic_t *compound_mapcount_ptr(struct page *page)
> -{
> -	return &page[1].compound_mapcount;
> -}
> -
> -static inline atomic_t *compound_pincount_ptr(struct page *page)
> -{
> -	return &page[2].hpage_pinned_refcount;
> -}
> +#define compound_mapcount_ptr(page)	(&(page)[1].compound_mapcount)
> +#define compound_pincount_ptr(page)	(&(page)[2].hpage_pinned_refcount)

Looks like this conversion is not covered by the reason given in the
commit message. Hm?

> +#define compound_head(page) ({						\
> +	__typeof__(page) _page = page;					\
> +	unsigned long head = READ_ONCE(_page->compound_head);		\
> +	if (unlikely(head & 1))						\
> +		_page = (void *)(head - 1);				\
> +	_page;								\
> +})

Ugh..

Other option would be to make compound_head() take 'const struct page *'
and return 'void *'. It arguably would provide better type safety, no?

-- 
 Kirill A. Shutemov