Update README.md

README.md

@@ -71,18 +71,16 @@ impl MappingBackend<MockFlags, MockPageTable> for MockBackend {
         &self,
         start: VirtAddr,
         size: usize,
-        old_flags: MockFlags,
         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);
+    ) -> bool {
         for entry in pt.iter_mut().skip(start.as_usize()).take(size) {
-            *entry = flags;
+            if *entry == 0 {
+                return false;
+            }
+            *entry = new_flags;
         }
-        Some(flags)
+        true
     }
 }
 ```