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

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 14 Aug 2025 14:47:10 +0800
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(-)


Does this need any re-evaluation after Willy's series?[1]  Patch 2/
changes page_to_section() to memdesc_section() which takes a new
memdesc_flags_t, ie. page->flags.  The conversion appears trivial, but
mm has many subtleties.

Ideally we could also avoid merge-time fixups for linux-next and
mainline.

Andrew, are you open to topic branch for Willy's series that I could
merge into vfio/next when it's considered stable?  Thanks,

Alex

[1]https://lore.kernel.org/all/20250805172307.1302730-3-willy@infradead.org/T/#u

> 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.
> + *
> + * Returns the number of contiguous pages.
> + */
> +static inline size_t num_pages_contiguous(struct page **pages, size_t nr_pages)
> +{
> +	struct page *cur_page = pages[0];
> +	unsigned long section = page_to_section(cur_page);
> +	size_t i;
> +
> +	for (i = 1; i < nr_pages; i++) {
> +		if (++cur_page != pages[i])
> +			break;
> +		/*
> +		 * In unproblematic kernel configs, page_to_section() == 0 and
> +		 * the whole check will get optimized out.
> +		 */
> +		if (page_to_section(cur_page) != section)
> +			break;
> +	}
> +
> +	return i;
> +}
> +
>  #endif