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

[lizhe.67@bytedance.com]

On Wed, 27 Aug 2025 12:10:55 -0600, alex.williamson@redhat.com wrote:

> 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.

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