dax: Convert dax_layout_busy_page to XArray

Instead of using a pagevec, just use the XArray iterators.  Add a
conditional rescheduling point which probably should have been there in
the original.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/dax.c

@@ -722,11 +722,10 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
  */
 struct page *dax_layout_busy_page(struct address_space *mapping)
 {
-	pgoff_t	indices[PAGEVEC_SIZE];
+	XA_STATE(xas, &mapping->i_pages, 0);
+	void *entry;
+	unsigned int scanned = 0;
 	struct page *page = NULL;
-	struct pagevec pvec;
-	pgoff_t	index, end;
-	unsigned i;
 
 	/*
 	 * In the 'limited' case get_user_pages() for dax is disabled.
@@ -737,13 +736,9 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
 	if (!dax_mapping(mapping) || !mapping_mapped(mapping))
 		return NULL;
 
-	pagevec_init(&pvec);
-	index = 0;
-	end = -1;
-
 	/*
 	 * If we race get_user_pages_fast() here either we'll see the
-	 * elevated page count in the pagevec_lookup and wait, or
+	 * elevated page count in the iteration and wait, or
 	 * get_user_pages_fast() will see that the page it took a reference
 	 * against is no longer mapped in the page tables and bail to the
 	 * get_user_pages() slow path.  The slow path is protected by
@@ -755,43 +750,26 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
 	 */
 	unmap_mapping_range(mapping, 0, 0, 1);
 
-	while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
-				min(end - index, (pgoff_t)PAGEVEC_SIZE),
-				indices)) {
-		for (i = 0; i < pagevec_count(&pvec); i++) {
-			struct page *pvec_ent = pvec.pages[i];
-			void *entry;
-
-			index = indices[i];
-			if (index >= end)
-				break;
-
-			if (WARN_ON_ONCE(!xa_is_value(pvec_ent)))
+	xas_lock_irq(&xas);
+	xas_for_each(&xas, entry, ULONG_MAX) {
+		if (WARN_ON_ONCE(!xa_is_value(entry)))
 			continue;
-
-			xa_lock_irq(&mapping->i_pages);
-			entry = get_unlocked_mapping_entry(mapping, index, NULL);
+		if (unlikely(dax_is_locked(entry)))
+			entry = get_unlocked_entry(&xas);
 		if (entry)
 			page = dax_busy_page(entry);
-			put_unlocked_mapping_entry(mapping, index, entry);
-			xa_unlock_irq(&mapping->i_pages);
+		put_unlocked_entry(&xas, entry);
 		if (page)
 			break;
-		}
-
-		/*
-		 * We don't expect normal struct page entries to exist in our
-		 * tree, but we keep these pagevec calls so that this code is
-		 * consistent with the common pattern for handling pagevecs
-		 * throughout the kernel.
-		 */
-		pagevec_remove_exceptionals(&pvec);
-		pagevec_release(&pvec);
-		index++;
+		if (++scanned % XA_CHECK_SCHED)
+			continue;
 
-		if (page)
-			break;
+		xas_pause(&xas);
+		xas_unlock_irq(&xas);
+		cond_resched();
+		xas_lock_irq(&xas);
 	}
+	xas_unlock_irq(&xas);
 	return page;
 }
 EXPORT_SYMBOL_GPL(dax_layout_busy_page);