On Fri, 5 Mar 2021 04:18:37 +0000 "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
> 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?
