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

David Hildenbrand
2025-08-21T23:17:08Z

On 21.08.25 22:55, Zi Yan wrote:
> 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.

Yeah, it's even called "n" in the kernel docs ...

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

... which we already have! (see above the macro) :)

Thanks!

--
Cheers

David / dhildenb