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

[lizhe.67@bytedance.com]

On Thu, 14 Aug 2025 08:54:44 +0200, david@redhat.com wrote:

> 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().
> "

Thank you for the reminder! The comment here should be revised as
follows:

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

Thanks,
Zhe