diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -206,11 +206,11 @@ def persist_nodemap(tr, revlog, pending=
                 fd.write(data)
                 if feed_data:
                     if use_mmap:
+                        fd.flush()
+                        new_data = util.buffer(util.mmapread(fd, new_length))
+                    else:
                         fd.seek(0)
                         new_data = fd.read(new_length)
-                    else:
-                        fd.flush()
-                        new_data = util.buffer(util.mmapread(fd, new_length))
             target_docket.data_length = new_length
             target_docket.data_unused = new_unused