Re: [PATCH v5 1/5] mm: introduce num_pages_contiguous()

[David Hildenbrand <david@redhat.com>]

On 14.08.25 08:47, lizhe.67@bytedance.com wrote:
> From: Li Zhe <lizhe.67@bytedance.com>
> 
> Let's add a simple helper for determining the number of contiguous pages
> that represent contiguous PFNs.
> 
> In an ideal world, this helper would be simpler or not even required.
> Unfortunately, on some configs we still have to maintain (SPARSEMEM
> without VMEMMAP), the memmap is allocated per memory section, and we might
> run into weird corner cases of false positives when blindly testing for
> contiguous pages only.
> 
> One example of such false positives would be a memory section-sized hole
> that does not have a memmap. The surrounding memory sections might get
> "struct pages" that are contiguous, but the PFNs are actually not.
> 
> This helper will, for example, be useful for determining contiguous PFNs
> in a GUP result, to batch further operations across returned "struct
> page"s. VFIO will utilize this interface to accelerate the VFIO DMA map
> process.
> 
> Implementation based on Linus' suggestions to avoid new usage of
> nth_page() where avoidable.
> 
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   include/linux/mm.h        |  7 ++++++-
>   include/linux/mm_inline.h | 35 +++++++++++++++++++++++++++++++++++
>   2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1ae97a0b8ec7..ead6724972cf 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1763,7 +1763,12 @@ static inline unsigned long page_to_section(const struct page *page)
>   {
>   	return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
>   }
> -#endif
> +#else /* !SECTION_IN_PAGE_FLAGS */
> +static inline unsigned long page_to_section(const struct page *page)
> +{
> +	return 0;
> +}
> +#endif /* SECTION_IN_PAGE_FLAGS */
>   
>   /**
>    * folio_pfn - Return the Page Frame Number of a folio.
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index 89b518ff097e..5ea23891fe4c 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -616,4 +616,39 @@ static inline bool vma_has_recency(struct vm_area_struct *vma)
>   	return true;
>   }
>   
> +/**
> + * num_pages_contiguous() - determine the number of contiguous pages
> + *			    that represent contiguous PFNs
> + * @pages: an array of page pointers
> + * @nr_pages: length of the array, at least 1
> + *
> + * Determine the number of contiguous pages that represent contiguous PFNs
> + * in @pages, starting from the first page.
> + *
> + * In kernel configs where contiguous pages might not imply contiguous PFNs
> + * over memory section boundaries, this function will stop at the memory
 > + * section boundary.

Jason suggested here instead:

"
In some kernel configs contiguous PFNs will not have contiguous struct
pages. In these configurations num_pages_contiguous() will return a
smaller than ideal number. The caller should continue to check for pfn
contiguity after each call to num_pages_contiguous().
"

-- 
Cheers

David / dhildenb