mm/gup: drop nth_page() usage in unpin_user_page_range_dirty_lock()

There is the concern that unpin_user_page_range_dirty_lock() might do some
weird merging of PFN ranges -- either now or in the future -- such that
PFN range is contiguous but the page range might not be.

Let's sanity-check for that and drop the nth_page() usage.

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

mm/gup.c

···
 static inline struct folio *gup_folio_range_next(struct page *start,
 		unsigned long npages, unsigned long i, unsigned int *ntails)
 {
-	struct page *next = nth_page(start, i);
+	struct page *next = start + i;
 	struct folio *folio = page_folio(next);
 	unsigned int nr = 1;
 
···
  * "gup-pinned page range" refers to a range of pages that has had one of the
  * pin_user_pages() variants called on that page.
  *
+ * The page range must be truly physically contiguous: the page range
+ * corresponds to a contiguous PFN range and all pages can be iterated
+ * naturally.
+ *
  * For the page ranges defined by [page .. page+npages], make that range (or
  * its head pages, if a compound page) dirty, if @make_dirty is true, and if the
  * page range was previously listed as clean.
···
 	unsigned long i;
 	struct folio *folio;
 	unsigned int nr;
+
+	VM_WARN_ON_ONCE(!page_range_contiguous(page, npages));
 
 	for (i = 0; i < npages; i += nr) {
 		folio = gup_folio_range_next(page, npages, i, &nr);