[PATCH v2 24/37] 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. As sg_set_page() gets inlined into modules, we have to
export the page_range_contiguous() helper -- use EXPORT_SYMBOL, there is
nothing special about this helper such that we would want to enforce
GPL-only modules.

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

Acked-by: Marek Szyprowski <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
---
 include/linux/scatterlist.h | 3 ++-
 mm/util.c                   | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 6f8a4965f9b98..29f6ceb98d74b 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -158,6 +158,7 @@ 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 +601,7 @@ 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;
 }
 
 /**
diff --git a/mm/util.c b/mm/util.c
index fbdb73aaf35fe..bb4b47cd67091 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1314,5 +1314,6 @@ bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
 			return false;
 	return true;
 }
+EXPORT_SYMBOL(page_range_contiguous);
 #endif
 #endif /* CONFIG_MMU */
-- 
2.50.1