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Crossing our fingers and hoping that short-term GUP will > have gone away isn't enough. To be clear it is not crossing fingers. If the page refcount is 0 then there are no references to that memory anywhere at all. It is 100% certain. It may take time to reach zero, but when it does it is safe. Many things rely on this property, including FSDAX. > For non-CoCo VMs, I expect we'll want to be much more permissive, but I think > they'll be a complete non-issue because there is no shared vs. private to worry > about. We can simply allow any and all userspace mappings for guest_memfd that is > attached to a "regular" VM, because a misbehaving userspace only loses whatever > hardening (or other benefits) was being provided by using guest_memfd. I.e. the > kernel and system at-large isn't at risk. It does seem to me like guest_memfd should really focus on the private aspect. If we need normal memfd enhancements of some kind to work better with KVM then that may be a better option than turning guest_memfd into memfd. Jason