For rr (https://rr-project.org) to support recording and replaying applications that use userfaultfd, we need to observe that a task we are controlling has blocked on a userfault. Currently this is very difficult to do, especially if a task blocks on a userfault on a page where some other task has already triggered a userfault, so no new userfaultfd event is generated. We also need to observe which page has been faulted on so we can determine when the fault has been serviced and the task is ready to run again. I've tried to find workarounds with existing APIs and it doesn't seem tractable. See https://github.com/rr-debugger/rr/issues/2852#issuecomment-837514946 for some thoughts about that. It seems to me that a sufficient API for us would be a new software perf event, e.g. PERF_COUNT_SW_USERFAULTS, with an associated PERF_SAMPLE_ADDR that would give us the address of the page. Does that sounds like a reasonable thing to add? Robert O'Callahan
