On Wed, 27 Aug 2025 12:10:55 -0600, alex.williamson@xxxxxxxxxx wrote:
> 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.
Thank you for your reminder.
In my view, if Willy's series is integrated, this patch will need to
be revised as follows. Please correct me if I'm wrong.
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ab4d979f4eec..bad0373099ad 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1763,7 +1763,12 @@ static inline unsigned long memdesc_section(memdesc_flags_t mdf)
{
return (mdf.f >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
}
-#endif
+#else /* !SECTION_IN_PAGE_FLAGS */
+static inline unsigned long memdesc_section(memdesc_flags_t mdf)
+{
+ 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 150302b4a905..bb23496d465b 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -616,4 +616,40 @@ 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 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.
+ */
+static inline size_t num_pages_contiguous(struct page **pages, size_t nr_pages)
+{
+ struct page *cur_page = pages[0];
+ unsigned long section = memdesc_section(cur_page->flags);
+ 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 (memdesc_section(cur_page->flags) != section)
+ break;
+ }
+
+ return i;
+}
+
#endif
---
Thanks,
Zhe
