Comment by anonymousDan
3 years ago
What is the actual perf benefit of userfaultd (e.g. for write protection)? It sounds interesting but its unclear to me why it would be any faster than a signal handler. Is it just a simpler code path within the kernel? Or is it that the hardware is configured to directly jump to a user space handler without any kernel intervention?
Well page protection is expensive which is why the predominant way to implement concurrent copying/compaction until recently was using read barriers (and still is -- ART seems to be an exception). I will mention that concurrent compacting GCs wouldn't use userfaultfd for write protection, but for read protection (it effectively takes over the job of a read barrier).
I personally don't know too much about userfaultfd and how it works internally, but my best guess is that it bypasses heavyweight kernel data-structures and lets the user application handle the page fault. It is obviously better than simply using mprotect, but it is not immediately clear why it would be better than a read barrier (other than code size considerations, which honestly doesn't sound like much of a big deal as the code handling userfaultfd also needs to be brought into the instruction cache).
I did find this kernel doc about userfaultfd [1] which might be interesting to read if you're interested (it also does mention that userfaultfd doesn't lock some internal kernel data-structures which gives it a better performance than simply using mprotect, but the implementation details are a bit sparse).
[1]: https://docs.kernel.org/admin-guide/mm/userfaultfd.html