Sharing page tables with msharefs

Perhaps they want to COW some regions of memory while sharing others. This is quite difficult to do without fork/clone because as far as I can tell there's no other good way to create a COW mapping of another COW mapping. (On windows this is completely impossible, which is why you can't easily do things like forking game saves without implementing COW as part of your application's data structures). I'm not 100% sure how to do this on linux (without using fork/clone), maybe you can reflink tempfs files together or something.