Oh neat, a post I actually know something about! I worked a lot on userfaultfd performance for GCE's live migration post-copy a couple years ago. Or more specifically, I worked on mechanisms to avoid it entirely- due to lock contention in the kennel, faults become veeeerry slow as the number of vcpus scales, and as it happens VMs these days can have a lot of vcpus

> Userfaultfd is a Linux mechanism, available since kernel 4.3, with additional event features like non-cooperative mode and fork/remap/remove tracking added in 4.11, that lets a userspace thread intercept and handle page faults.

Is this the same feature Windows has had forever, or is there more to it?