On Thu, Aug 28, 2025 at 12:01:27AM +0200, 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. 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.
Ah you mention this here (I wrote end of this first :)
>
> We can now drop the nth_page() usage in sg_page_iter_page().
>
> Acked-by: Marek Szyprowski <m.szyprow...@samsung.com>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
All LGTM, so:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoa...@oracle.com>
> ---
> 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));
This is pretty horrible as one statement, but I guess we can't really do better,
I had a quick look around for some helper that could work but nothing is clearly
suitable.
So this should be fine.
> 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 0bf349b19b652..e8b9da6b13230 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1312,5 +1312,6 @@ bool page_range_contiguous(const struct page *page,
> unsigned long nr_pages)
> return false;
> return true;
> }
> +EXPORT_SYMBOL(page_range_contiguous);
Kinda sad that we're doing this as EXPORT_SYMBOL() rather than
EXPORT_SYMBOL_GPL() :( but I guess necessary to stay consistent...
> #endif
> #endif /* CONFIG_MMU */
> --
> 2.50.1
>
