On 05.08.25 03:24, Alex Williamson wrote:
Objections were raised to adding this helper to common code with only a
single user and dubious generalism. Pull it back into subsystem code.
Link: https://lore.kernel.org/all/CAHk-=whhYRMS7Xc9k_JBdrGvp++JLmU0T2xXEgn046hWrj7q8Q@xxxxxxxxxxxxxx/
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
---
So, I took the original patch and
* moved the code to mm_inline.h (sounds like a better fit)
* Tweaked the patch description
* Tweaked the documentation and turned it into proper kerneldoc
* Made the function return "size_t" as well
* Use the page_to_section() trick to avoid nth_page().
Only compile-tested so far. Still running it through some cross compiles.
From 36d67849bfdbc184990f21464c53585d35648616 Mon Sep 17 00:00:00 2001
From: Li Zhe <lizhe.67@xxxxxxxxxxxxx>
Date: Thu, 10 Jul 2025 16:53:51 +0800
Subject: [PATCH] mm: introduce num_pages_contiguous()
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(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index fa538feaa8d95..2852bcd792745 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1759,7 +1759,12 @@ 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 89b518ff097e6..58cb99b69f432 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
--
2.50.1
--
Cheers,
David / dhildenb
