Semantically this should do the job: anonymous memory -> vmsplice(gift) -> pipe -> splice(move) -> memfd. Then remap the pages from memfd to the original addres

Semantically this should do the job: anonymous memory -> vmsplice(gift) -> pipe -> splice(move) -> memfd. Then remap the pages from memfd to the original address ranges so you're allowed to use them again. But I don't know whether this is supported on memfds. And of course it's not atomic.