linux-kernel - Re: [PATCH RFC 13/35] mm: simplify folio_page() and folio_page

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Message-ID: <23c6e511-19b2-4662-acfc-18692c899a6c@redhat.com>
Date: Thu, 21 Aug 2025 23:00:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
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 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
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 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
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 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org
Subject: Re: [PATCH RFC 13/35] mm: simplify folio_page() and folio_page_idx()

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@...hat.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