(I am the person you are replying to)

It could work like this. "Hey OS I would like to process these pages* are they good to go? If not could you fetch and lock them for me" and then if they are ready you process them knowing it won't fault, and if they are not you do something else and try again later.

It's a sort of hybrid of the mmap and fread paradigms in that there are both explicit read requests but the kernel can also get you data on its own initiative if there are spare resources for it.

* to amortize syscall overhead.