Re: [PATCH 01/11] mm: Change the interface of prep_compound_tail()

[Usama Arif <usamaarif642@gmail.com>]

On 05/12/2025 19:43, Kiryl Shutsemau wrote:
> Instead of passing down the head page and tail page index, pass the tail
> and head pages directly, as well as the order of the compound page.
> 
> This is a preparation for changing how the head position is encoded in
> the tail page.
> 
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> ---
>  include/linux/page-flags.h |  4 +++-
>  mm/hugetlb.c               |  8 +++++---
>  mm/internal.h              | 11 +++++------
>  mm/mm_init.c               |  2 +-
>  mm/page_alloc.c            |  2 +-
>  5 files changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 0091ad1986bf..2c1153dd7e0e 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -865,7 +865,9 @@ static inline bool folio_test_large(const struct folio *folio)
>  	return folio_test_head(folio);
>  }
>  
> -static __always_inline void set_compound_head(struct page *page, struct page *head)
> +static __always_inline void set_compound_head(struct page *page,
> +					      struct page *head,
> +					      unsigned int order)

I can see that order is used later, I think patch 4, but probably this patch might cause a
build warning as order is unused? Might be good to integrate that into the later patch?

Other nit is, do we want const for head here? (Its not there before, but might be good to add).

>  {
>  	WRITE_ONCE(page->compound_head, (unsigned long)head + 1);
>  }
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 0455119716ec..a55d638975bd 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3212,6 +3212,7 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
>  
>  /* Initialize [start_page:end_page_number] tail struct pages of a hugepage */
>  static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
> +					struct hstate *h,
>  					unsigned long start_page_number,
>  					unsigned long end_page_number)
>  {
> @@ -3220,6 +3221,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
>  	struct page *page = folio_page(folio, start_page_number);
>  	unsigned long head_pfn = folio_pfn(folio);
>  	unsigned long pfn, end_pfn = head_pfn + end_page_number;
> +	unsigned int order = huge_page_order(h);
>  
>  	/*
>  	 * As we marked all tail pages with memblock_reserved_mark_noinit(),
> @@ -3227,7 +3229,7 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
>  	 */
>  	for (pfn = head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) {
>  		__init_single_page(page, pfn, zone, nid);
> -		prep_compound_tail((struct page *)folio, pfn - head_pfn);
> +		prep_compound_tail(page, &folio->page, order);
>  		set_page_count(page, 0);
>  	}
>  }
> @@ -3247,7 +3249,7 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
>  	__folio_set_head(folio);
>  	ret = folio_ref_freeze(folio, 1);
>  	VM_BUG_ON(!ret);
> -	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
> +	hugetlb_folio_init_tail_vmemmap(folio, h, 1, nr_pages);
>  	prep_compound_head((struct page *)folio, huge_page_order(h));
>  }
>  
> @@ -3304,7 +3306,7 @@ static void __init prep_and_add_bootmem_folios(struct hstate *h,
>  			 * time as this is early in boot and there should
>  			 * be no contention.
>  			 */
> -			hugetlb_folio_init_tail_vmemmap(folio,
> +			hugetlb_folio_init_tail_vmemmap(folio, h,
>  					HUGETLB_VMEMMAP_RESERVE_PAGES,
>  					pages_per_huge_page(h));
>  		}
> diff --git a/mm/internal.h b/mm/internal.h
> index 1561fc2ff5b8..0355da7cb6df 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -810,13 +810,12 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
>  		INIT_LIST_HEAD(&folio->_deferred_list);
>  }
>  
> -static inline void prep_compound_tail(struct page *head, int tail_idx)
> +static inline void prep_compound_tail(struct page *tail,
> +				      struct page *head, unsigned int order)
>  {
> -	struct page *p = head + tail_idx;
> -
> -	p->mapping = TAIL_MAPPING;
> -	set_compound_head(p, head);
> -	set_page_private(p, 0);
> +	tail->mapping = TAIL_MAPPING;
> +	set_compound_head(tail, head, order);
> +	set_page_private(tail, 0);
>  }
>  
>  void post_alloc_hook(struct page *page, unsigned int order, gfp_t gfp_flags);
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 7712d887b696..87d1e0277318 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1102,7 +1102,7 @@ static void __ref memmap_init_compound(struct page *head,
>  		struct page *page = pfn_to_page(pfn);
>  
>  		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
> -		prep_compound_tail(head, pfn - head_pfn);
> +		prep_compound_tail(page, head, order);
>  		set_page_count(page, 0);
>  	}
>  	prep_compound_head(head, order);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ed82ee55e66a..fe77c00c99df 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -717,7 +717,7 @@ void prep_compound_page(struct page *page, unsigned int order)
>  
>  	__SetPageHead(page);
>  	for (i = 1; i < nr_pages; i++)
> -		prep_compound_tail(page, i);
> +		prep_compound_tail(page + i, page, order);
>  
>  	prep_compound_head(page, order);
>  }