diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c76f6ea..e8eebe7 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2809,7 +2809,7 @@
 
 	fs_info->bdi.ra_pages *= btrfs_super_num_devices(disk_super);
 	fs_info->bdi.ra_pages = max(fs_info->bdi.ra_pages,
-				    4 * 1024 * 1024 / PAGE_CACHE_SIZE);
+				    SZ_4M / PAGE_CACHE_SIZE);
 
 	tree_root->nodesize = nodesize;
 	tree_root->sectorsize = sectorsize;