On Thu, 14 Aug 2025 14:47:10 +0800 lizhe.67@xxxxxxxxxxxxx wrote: > From: Li Zhe <lizhe.67@xxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxxxxx> > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxx> > Signed-off-by: Li Zhe <lizhe.67@xxxxxxxxxxxxx> > Co-developed-by: David Hildenbrand <david@xxxxxxxxxx> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> > --- > 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@xxxxxxxxxxxxx/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
