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)
