diff -r 0314e02efa25 -r ca5b833ce756 mercurial/configitems.py
--- a/mercurial/configitems.py	Sun Oct 08 20:42:19 2017 +0200
+++ b/mercurial/configitems.py	Sun Oct 08 20:43:46 2017 +0200
@@ -197,6 +197,9 @@
 coreconfigitem('experimental', 'maxdeltachainspan',
     default=-1,
 )
+coreconfigitem('experimental', 'mmapindexthreshold',
+    default=None,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )