[wmf]

If C had exceptions a page fault could safely unwind the stack up to the main loop which could work on something else until the page arrives. This has the advantage that there's no cost for the common case of accessing resident pages. Exceptions seem to have fallen out of favor so this may trade one problem for another.

[vlovich123]

C++ has exceptions and having seen the vast majority of code and the way it’s written and the understanding of people writing it, exception safety is a foreign concept. Doing it in C without RAII seems particularly masochistic and doomed to fail.

And unwinding the stack isn’t what you want to do because you’re basically signaling you want to cancel the operation and you’re throwing all the state when you precisely don’t want to do that - you just want to pause the current task and do other I/O in the meantime.

[gpderetta]

you can longjmp, swapcontext or whatever from a signal handler into another lightweight fiber. The problem is that there is no "until the page arrive" notification. You would have to poll mincore which is awful.

You could of course imagine an ansychronous "mmap complete notification" syscal, but at that point why not just use io_uring, it will be simpler and it has the benefit of actually existing.

[pjmlp]

Windows C has exceptions, and no one has ever thought about doing something like this.

They are only used for the same purpose as UNIX signals, without their flaws.

In any case, page faults are OS specific, how to standardise such behaviour, with the added performance loss switching between both userspace and kernel.