diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 5a7e60d..8935eec 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -159,7 +159,7 @@
 int csum_dirty_buffer(struct btrfs_root *root, struct page *page)
 {
 	struct extent_map_tree *tree;
-	u64 start = page->index << PAGE_CACHE_SHIFT;
+	u64 start = (u64)page->index << PAGE_CACHE_SHIFT;
 	u64 found_start;
 	int found_level;
 	unsigned long len;