Re: [PATCH RFC 13/35] mm: simplify folio_page() and folio_page_idx()

Zi Yan
2025-08-21T23:19:54Z

On 21 Aug 2025, at 16:06, David Hildenbrand wrote:

> Now that a single folio/compound page can no longer span memory sections
> in problematic kernel configurations, we can stop using nth_page().
>
> While at it, turn both macros into static inline functions and add
> kernel doc for folio_page_idx().
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> include/linux/mm.h | 16 ++++++++++++++--
> include/linux/page-flags.h | 5 ++++-
> 2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 48a985e17ef4e..ef360b72cb05c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -210,10 +210,8 @@ extern unsigned long sysctl_admin_reserve_kbytes;
>
> #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
> -#define folio_page_idx(folio, p) (page_to_pfn(p) - folio_pfn(folio))
> #else
> #define nth_page(page,n) ((page) + (n))
> -#define folio_page_idx(folio, p) ((p) - &(folio)->page)
> #endif
>
> /* to align the pointer to the (next) page boundary */
> @@ -225,6 +223,20 @@ extern unsigned long sysctl_admin_reserve_kbytes;
> /* test whether an address (unsigned long or pointer) is aligned to PAGE_SIZE */
> #define PAGE_ALIGNED(addr) IS_ALIGNED((unsigned long)(addr), PAGE_SIZE)
>
> +/**
> + * folio_page_idx - Return the number of a page in a folio.
> + * @folio: The folio.
> + * @page: The folio page.
> + *
> + * This function expects that the page is actually part of the folio.
> + * The returned number is relative to the start of the folio.
> + */
> +static inline unsigned long folio_page_idx(const struct folio *folio,
> + const struct page *page)
> +{
> + return page - &folio->page;
> +}
> +
> static inline struct folio *lru_to_folio(struct list_head *head)
> {
> return list_entry((head)->prev, struct folio, lru);
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index d53a86e68c89b..080ad10c0defc 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -316,7 +316,10 @@ static __always_inline unsigned long _compound_head(const struct page *page)
> * check that the page number lies within @folio; the caller is presumed
> * to have a reference to the page.
> */
> -#define folio_page(folio, n) nth_page(&(folio)->page, n)
> +static inline struct page *folio_page(struct folio *folio, unsigned long nr)
> +{
> + return &folio->page + nr;
> +}

Maybe s/nr/n/ or s/nr/nth/, since it returns the nth page within a folio.

Since you have added kernel doc for folio_page_idx(), it does not hurt
to have something similar for folio_page(). :)

+/**
+ * folio_page - Return the nth page in a folio.
+ * @folio: The folio.
+ * @n: Page index within the folio.
+ *
+ * This function expects that n does not exceed folio_nr_pages(folio).
+ * The returned page is relative to the first page of the folio.
+ */

>
> static __always_inline int PageTail(const struct page *page)
> {
> --
> 2.50.1

Otherwise, Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi