diff -r 51da7c8ef357 -r 05a1e44b2224 mercurial/configitems.py
--- a/mercurial/configitems.py	Thu Jan 14 01:12:42 2021 +0100
+++ b/mercurial/configitems.py	Thu Jan 14 01:25:06 2021 +0100
@@ -1793,6 +1793,14 @@
 coreconfigitem(
     b'storage', b'revlog.nodemap.mode', default=b'compat', experimental=True
 )
+# experimental as long as format.use-persistent-nodemap is.
+coreconfigitem(
+    b'storage',
+    b'revlog.persistent-nodemap.slow-path',
+    default=b"allow",
+    experimental=True,
+)
+
 coreconfigitem(
     b'storage',
     b'revlog.reuse-external-delta',