diff --git a/mercurial/configitems.toml b/mercurial/configitems.toml
--- a/mercurial/configitems.toml
+++ b/mercurial/configitems.toml
@@ -2147,7 +2147,7 @@ experimental = true  # experimental as l
 [[items]]
 section = "storage"
 name = "revbranchcache.mmap"
-default = true
+default = false
 
 [[items]]
 section = "storage"