Linux Page Faults, mmap, and userfaultfd

I recently went down a rabbit hole trying to understand how Linux handles page faults, what mmap actually does at the physical page level, and how userfaultfd lets userspace take over that fault handling. The motivation was a specific problem, which was making Virtual Machine (VM) snapshot restore fast by lazily populating guest memory. But the underlying mechanisms are general Linux concepts that I think are worth understanding on their own.