diff --git a/samples/memfs/memfs_test.go b/samples/memfs/memfs_test.go
index cc32073..b1f5181 100644
--- a/samples/memfs/memfs_test.go
+++ b/samples/memfs/memfs_test.go
@@ -35,7 +35,9 @@ import (
 
 func TestMemFS(t *testing.T) { RunTests(t) }
 
-// TODO(jacobsa): Comments.
+// The radius we use for "expect mtime is within"-style assertions. We can't
+// share a synchronized clock with the ultimate source of mtimes because with
+// writeback caching enabled the kernel manufactures them based on wall time.
 const timeSlop = 5 * time.Millisecond
 
 ////////////////////////////////////////////////////////////////////////