diff --git a/README.md b/README.md
index dac9f86..bff3941 100644
--- a/README.md
+++ b/README.md
@@ -75,6 +75,9 @@ impl MappingBackend<MockFlags, MockPageTable> for MockBackend {
         new_flags: MockFlags,
         pt: &mut MockPageTable,
     ) -> Option<MockFlags> {
+        if (old_flags & 0x7) == (new_flags & 0x7) {
+            return None;
+        }
         let flags = (new_flags & 0x7) | (old_flags & !0x7);
         for entry in pt.iter_mut().skip(start.as_usize()).take(size) {
             *entry = flags;