diff -r 683b96c416d8 -r 027bc364524b mercurial/localrepo.py
--- a/mercurial/localrepo.py	Tue Oct 10 10:03:42 2023 +0200
+++ b/mercurial/localrepo.py	Tue Oct 10 10:03:50 2023 +0200
@@ -1118,7 +1118,7 @@
 
     mmapindexthreshold = ui.configbytes(b'experimental', b'mmapindexthreshold')
     if mmapindexthreshold is not None:
-        options[b'mmapindexthreshold'] = mmapindexthreshold
+        data_config.mmap_index_threshold = mmapindexthreshold
 
     withsparseread = ui.configbool(b'experimental', b'sparse-read')
     srdensitythres = float(