page cache: Convert find_get_entry to XArray

Slightly shorter and simpler code.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

mm/filemap.c

@@ -1382,27 +1382,21 @@ EXPORT_SYMBOL(page_cache_prev_miss);
  */
 struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
 {
-	void **pagep;
+	XA_STATE(xas, &mapping->i_pages, offset);
 	struct page *head, *page;
 
 	rcu_read_lock();
 repeat:
-	page = NULL;
-	pagep = radix_tree_lookup_slot(&mapping->i_pages, offset);
-	if (pagep) {
-		page = radix_tree_deref_slot(pagep);
-		if (unlikely(!page))
-			goto out;
-		if (radix_tree_exception(page)) {
-			if (radix_tree_deref_retry(page))
+	xas_reset(&xas);
+	page = xas_load(&xas);
+	if (xas_retry(&xas, page))
 		goto repeat;
 	/*
-			 * A shadow entry of a recently evicted page,
-			 * or a swap entry from shmem/tmpfs.  Return
-			 * it without attempting to raise page count.
+	 * A shadow entry of a recently evicted page, or a swap entry from
+	 * shmem/tmpfs.  Return it without attempting to raise page count.
 	 */
+	if (!page || xa_is_value(page))
 		goto out;
-		}
 
 	head = compound_head(page);
 	if (!page_cache_get_speculative(head))
@@ -1419,11 +1413,10 @@ struct page *find_get_entry(struct address_space *mapping, pgoff_t offset)
 	 * This is part of the lockless pagecache protocol. See
 	 * include/linux/pagemap.h for details.
 	 */
-		if (unlikely(page != *pagep)) {
+	if (unlikely(page != xas_reload(&xas))) {
 		put_page(head);
 		goto repeat;
 	}
-	}
 out:
 	rcu_read_unlock();
 
@@ -1453,7 +1446,7 @@ struct page *find_lock_entry(struct address_space *mapping, pgoff_t offset)
 
 repeat:
 	page = find_get_entry(mapping, offset);
-	if (page && !radix_tree_exception(page)) {
+	if (page && !xa_is_value(page)) {
 		lock_page(page);
 		/* Has the page been truncated? */
 		if (unlikely(page_mapping(page) != mapping)) {