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().

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Marek Szyprowski <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/scatterlist.h

···
 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;
···
  */
 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;
 }
 
 /**

mm/util.c

···
 			return false;
 	return true;
 }
+EXPORT_SYMBOL(page_range_contiguous);
 #endif