[PATCH v7 02/28] mm: Introduce struct folio

[Matthew Wilcox]

On Fri, Apr 09, 2021 at 07:50:39PM +0100, Matthew Wilcox (Oracle) wrote:
> A struct folio is a new abstraction to replace the venerable struct page.
> A function which takes a struct folio argument declares that it will
> operate on the entire (possibly 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.
> +++ b/include/linux/mm_types.h
[...]
> +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;
> +}

I'm looking at changing this for the next revision, and basing it on
my recent patch to make compound_head() const-preserving:

+#define page_folio(page)       _Generic((page),                        \
+       const struct page *:    (const struct folio *)_compound_head(page), \
+       struct page *:          (struct folio *)_compound_head(page))

I've also noticed an awkward pattern occurring that I think this makes
less awkward:

+/**
+ * folio_page - Return a page from a folio.
+ * @folio: The folio.
+ * @n: The page number to return.
+ *
+ * @n is relative to the start of the folio.  It should be between
+ * 0 and folio_nr_pages(@folio) - 1, but this is not checked for.
+ */
+#define folio_page(folio, n)   nth_page(&(folio)->page, n)

That lets me simplify folio_next():

+static inline struct folio *folio_next(struct folio *folio)
+{
+       return (struct folio *)folio_page(folio, folio_nr_pages(folio));
+}

(it occurs to me this should also be const-preserving, but it's not clear
that's needed yet)