You’ve actually understood my suggestion - thank you. Unfortunately I think hint_read inherently can’t work because it’s a race condition between the read and how long you access the page. And this race is inherent in any attempted solution that needs to be solved. Signals are also the wrong abstraction mechanism (and are slow and have all sorts of other problems).
You need something more complicated I think, like rseq and futex you have some shared data structure that both understand how to mutate atomically. You could literally use rseq to abort if the page isn’t in memory and then submit an io_uring task to get signaled when it gets paged in again but rseq is a bit too coarse (it’ll trigger on any preemption).
There’s a race condition starvation danger here (it gets evicted between when you get the signal and the sequence completes) but something like this conceptually could maybe be closer to working.
But yes it’s inherently difficult which is why it doesn’t exist but it is higher performance. And yes, this only makes sense for mmap not all allocations so SIGSEGV is irrelevant if looking at today’s kernels.
You need something more complicated I think, like rseq and futex you have some shared data structure that both understand how to mutate atomically. You could literally use rseq to abort if the page isn’t in memory and then submit an io_uring task to get signaled when it gets paged in again but rseq is a bit too coarse (it’ll trigger on any preemption).
There’s a race condition starvation danger here (it gets evicted between when you get the signal and the sequence completes) but something like this conceptually could maybe be closer to working.
But yes it’s inherently difficult which is why it doesn’t exist but it is higher performance. And yes, this only makes sense for mmap not all allocations so SIGSEGV is irrelevant if looking at today’s kernels.