diff -r bf4a078b3f88 -r d82dd55024e7 mercurial/localrepo.py
--- a/mercurial/localrepo.py	Fri Jan 18 16:02:26 2019 +0100
+++ b/mercurial/localrepo.py	Fri Jan 18 16:03:37 2019 +0100
@@ -758,7 +758,8 @@
     if 0 <= chainspan:
         options[b'maxdeltachainspan'] = chainspan
 
-    mmapindexthreshold = ui.configbytes(b'storage', b'mmap-threshold')
+    mmapindexthreshold = ui.configbytes(b'experimental',
+                                        b'mmapindexthreshold')
     if mmapindexthreshold is not None:
         options[b'mmapindexthreshold'] = mmapindexthreshold