Except it’s not irreplaceable, at least on Linux. userfaultfd allows you to define custom page fault handling. With it, you can even do crazy things like “mmap” a remote resource by making HTTP range requests on a read fault.
You’re right. Irreplaceable was a stronger way of putting it than the original, I think, so that’s more my mistake than hers, and I think the contrast with userspace stands.
mmap sits at this lovely intersection between virtual memory and the disk, and it’s been around for a long time. By now there are other means of playing within that nice intersection, but mmap is the pop classic.
Windows has vectored exception handlers, which are a bit like UNIX signal handlers but much more sanely designed. You can use that to redirect control flow when a page fault occurs, and you can check the faulting address in your handler to scope it to a particular region of memory.
