page cache: Convert find_get_pages_contig to XArray

There's no direct replacement for radix_tree_for_each_contig()
in the XArray API as it's an unusual thing to do. Instead,
open-code a loop using xas_next(). This removes the only user of
radix_tree_for_each_contig() so delete the iterator from the API and
the test suite code for it.

Signed-off-by: Matthew Wilcox <[e-mail]>

.clang-format

···
   - 'protocol_for_each_card'
   - 'protocol_for_each_dev'
   - 'queue_for_each_hw_ctx'
-  - 'radix_tree_for_each_contig'
   - 'radix_tree_for_each_slot'
   - 'radix_tree_for_each_tagged'
   - 'rbtree_postorder_for_each_entry_safe'

include/linux/radix-tree.h

···
 	     slot = radix_tree_next_slot(slot, iter, 0))
 
 /**
- * radix_tree_for_each_contig - iterate over contiguous slots
- *
- * @slot:	the void** variable for pointer to slot
- * @root:	the struct radix_tree_root pointer
- * @iter:	the struct radix_tree_iter pointer
- * @start:	iteration starting index
- *
- * @slot points to radix tree slot, @iter->index contains its index.
- */
-#define radix_tree_for_each_contig(slot, root, iter, start)		\
-	for (slot = radix_tree_iter_init(iter, start) ;			\
-	     slot || (slot = radix_tree_next_chunk(root, iter,		\
-				RADIX_TREE_ITER_CONTIG)) ;		\
-	     slot = radix_tree_next_slot(slot, iter,			\
-				RADIX_TREE_ITER_CONTIG))
-
-/**
  * radix_tree_for_each_tagged - iterate over tagged slots
  *
  * @slot:	the void** variable for pointer to slot

mm/filemap.c

···
 unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 			       unsigned int nr_pages, struct page **pages)
 {
-	struct radix_tree_iter iter;
-	void **slot;
+	XA_STATE(xas, &mapping->i_pages, index);
+	struct page *page;
 	unsigned int ret = 0;
 
 	if (unlikely(!nr_pages))
 		return 0;
 
 	rcu_read_lock();
-	radix_tree_for_each_contig(slot, &mapping->i_pages, &iter, index) {
-		struct page *head, *page;
-repeat:
-		page = radix_tree_deref_slot(slot);
-		/* The hole, there no reason to continue */
-		if (unlikely(!page))
+	for (page = xas_load(&xas); page; page = xas_next(&xas)) {
+		struct page *head;
+		if (xas_retry(&xas, page))
+			continue;
+		/*
+		 * If the entry has been swapped out, we can stop looking.
+		 * No current caller is looking for DAX entries.
+		 */
+		if (xa_is_value(page))
 			break;
-
-		if (radix_tree_exception(page)) {
-			if (radix_tree_deref_retry(page)) {
-				slot = radix_tree_iter_retry(&iter);
-				continue;
-			}
-			/*
-			 * A shadow entry of a recently evicted page,
-			 * or a swap entry from shmem/tmpfs.  Stop
-			 * looking for contiguous pages.
-			 */
-			break;
-		}
 
 		head = compound_head(page);
 		if (!page_cache_get_speculative(head))
-			goto repeat;
+			goto retry;
 
 		/* The page was split under us? */
-		if (compound_head(page) != head) {
-			put_page(head);
-			goto repeat;
-		}
+		if (compound_head(page) != head)
+			goto put_page;
 
 		/* Has the page moved? */
-		if (unlikely(page != *slot)) {
-			put_page(head);
-			goto repeat;
-		}
+		if (unlikely(page != xas_reload(&xas)))
+			goto put_page;
 
 		/*
 		 * must check mapping and index after taking the ref.
 		 * otherwise we can get both false positives and false
 		 * negatives, which is just confusing to the caller.
 		 */
-		if (page->mapping == NULL || page_to_pgoff(page) != iter.index) {
+		if (!page->mapping || page_to_pgoff(page) != xas.xa_index) {
 			put_page(page);
 			break;
 		}
···
 		pages[ret] = page;
 		if (++ret == nr_pages)
 			break;
+		continue;
+put_page:
+		put_page(head);
+retry:
+		xas_reset(&xas);
 	}
 	rcu_read_unlock();
 	return ret;

tools/testing/radix-tree/regression3.c

···
 			continue;
 		}
 	}
-	radix_tree_delete(&root, 1);
-
-	first = true;
-	radix_tree_for_each_contig(slot, &root, &iter, 0) {
-		printv(2, "contig %ld %p\n", iter.index, *slot);
-		if (first) {
-			radix_tree_insert(&root, 1, ptr);
-			first = false;
-		}
-		if (radix_tree_deref_retry(*slot)) {
-			printv(2, "retry at %ld\n", iter.index);
-			slot = radix_tree_iter_retry(&iter);
-			continue;
-		}
-	}
 
 	radix_tree_for_each_slot(slot, &root, &iter, 0) {
 		printv(2, "slot %ld %p\n", iter.index, *slot);
-		if (!iter.index) {
-			printv(2, "next at %ld\n", iter.index);
-			slot = radix_tree_iter_resume(slot, &iter);
-		}
-	}
-
-	radix_tree_for_each_contig(slot, &root, &iter, 0) {
-		printv(2, "contig %ld %p\n", iter.index, *slot);
 		if (!iter.index) {
 			printv(2, "next at %ld\n", iter.index);
 			slot = radix_tree_iter_resume(slot, &iter);