Re: [PATCH v4 01/25] mm: Introduce struct folio

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  5 Mar 2021 04:18:37 +0000 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:

> A struct folio refers to an entire (possibly compound) page.  A function
> which takes a struct folio argument declares that it will operate on the
> entire compound page, not just PAGE_SIZE bytes.  In return, the caller
> guarantees that the pointer it is passing does not point to a tail page.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/mm.h       | 30 ++++++++++++++++++++++++++++++
>  include/linux/mm_types.h | 17 +++++++++++++++++

Perhaps a new folio.h would be neater.

> @@ -1518,6 +1523,30 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
>  #endif
>  }
>  
> +static inline unsigned long folio_nr_pages(struct folio *folio)
> +{
> +	return compound_nr(&folio->page);
> +}
> +
> +static inline struct folio *next_folio(struct folio *folio)
> +{
> +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> +	return (struct folio *)nth_page(&folio->page, folio_nr_pages(folio));
> +#else
> +	return folio + folio_nr_pages(folio);
> +#endif
> +}

It's a shame this isn't called folio_something(), like the rest of the API.

Unclear what this does.  Some comments would help.

> +static inline unsigned int folio_shift(struct folio *folio)
> +{
> +	return PAGE_SHIFT + folio_order(folio);
> +}
> +
> +static inline size_t folio_size(struct folio *folio)
> +{
> +	return PAGE_SIZE << folio_order(folio);
> +}

Why size_t?  That's pretty rare in this space and I'd have expected
unsigned long.


> @@ -1623,6 +1652,7 @@ extern void pagefault_out_of_memory(void);
>  
>  #define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK)
>  #define offset_in_thp(page, p)	((unsigned long)(p) & (thp_size(page) - 1))
> +#define offset_in_folio(folio, p) ((unsigned long)(p) & (folio_size(folio) - 1))
>  
>  /*
>   * Flags passed to show_mem() and show_free_areas() to suppress output in
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 0974ad501a47..a311cb48526f 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -223,6 +223,23 @@ struct page {
>  #endif
>  } _struct_page_alignment;
>  
> +/*
> + * A struct folio is either a base (order-0) page or the head page of
> + * a compound page.
> + */
> +struct folio {
> +	struct page page;
> +};
> +
> +static inline struct folio *page_folio(struct page *page)
> +{
> +	unsigned long head = READ_ONCE(page->compound_head);
> +
> +	if (unlikely(head & 1))
> +		return (struct folio *)(head - 1);
> +	return (struct folio *)page;
> +}

What purpose does the READ_ONCE() serve?