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

[Marek Szyprowski]

On 21.08.2025 22:06, David Hildenbrand wrote:
> 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 at redhat.com>
Acked-by: Marek Szyprowski <m.szyprowski at samsung.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;
>   }
>   
>   /**

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland