linux-kernel - [PATCH RFC 23/35] scatterlist: disallow non-contigous page ranges in a single SG entry

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Message-ID: <20250821200701.1329277-24-david@redhat.com>
Date: Thu, 21 Aug 2025 22:06:49 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: David Hildenbrand <david@...hat.com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
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Subject: [PATCH RFC 23/35] scatterlist: disallow non-contigous page ranges in a single SG entry

The expectation is that there is currently no user that would pass in
non-contigous page ranges: no allocator, not even VMA, will hand these
out.

The only problematic part would be if someone would provide a range
obtained directly from memblock, or manually merge problematic ranges.
If we find such cases, we should fix them to create separate
SG entries.

Let's check in sg_set_page() that this is really the case. No need to
check in sg_set_folio(), as pages in a folio are guaranteed to be
contiguous.

We can now drop the nth_page() usage in sg_page_iter_page().

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 include/linux/scatterlist.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 6f8a4965f9b98..8196949dfc82c 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -6,6 +6,7 @@
 #include <linux/types.h>
 #include <linux/bug.h>
 #include <linux/mm.h>
+#include <linux/mm_inline.h>
 #include <asm/io.h>
 
 struct scatterlist {
@@ -158,6 +159,7 @@ static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
 static inline void sg_set_page(struct scatterlist *sg, struct page *page,
 			       unsigned int len, unsigned int offset)
 {
+	VM_WARN_ON_ONCE(!page_range_contiguous(page, ALIGN(len + offset, PAGE_SIZE) / PAGE_SIZE));
 	sg_assign_page(sg, page);
 	sg->offset = offset;
 	sg->length = len;
@@ -600,7 +602,7 @@ void __sg_page_iter_start(struct sg_page_iter *piter,
  */
 static inline struct page *sg_page_iter_page(struct sg_page_iter *piter)
 {
-	return nth_page(sg_page(piter->sg), piter->sg_pgoffset);
+	return sg_page(piter->sg) + piter->sg_pgoffset;
 }
 
 /**
-- 
2.50.1