diff -r 97ebdb192b00 -r 6493f0a567c2 mercurial/configitems.py
--- a/mercurial/configitems.py	Sun Apr 05 18:32:46 2020 +0200
+++ b/mercurial/configitems.py	Tue Apr 14 02:45:05 2020 +0200
@@ -681,6 +681,9 @@
     b'experimental', b'exp-persistent-nodemap.mmap', default=True,
 )
 coreconfigitem(
+    b'experimental', b'exp-persistent-nodemap.mode', default=b'compat',
+)
+coreconfigitem(
     b'experimental', b'server.filesdata.recommended-batch-size', default=50000,
 )
 coreconfigitem(