On Thu, Feb 23, 2023 at 05:11:11PM +0000, Nadav Amit wrote: > From my experience with UFFD, proper ordering of events is crucial, although it > is not always done well. Therefore, we should aim for improvement, not > regression. I believe that utilizing the pagemap-based mechanism for WP'ing > might be a step in the wrong direction. I think that it would have been better > to emit a 'UFFD_FEATURE_WP_ASYNC' WP-log (and ordered) with UFFD #PF and > events. The 'UFFD_FEATURE_WP_ASYNC'-log may not need to wake waiters on the > file descriptor unless the log is full. Yes this is an interesting question to think about.. Keeping the data in the pgtable has one good thing that it doesn't need any complexity on maintaining the log, and no possibility of "log full". If there's possible "log full" then the next question is whether we should let the worker wait the monitor if the monitor is not fast enough to collect those data. It adds some slight dependency on the two threads, I think it can make the tracking harder or impossible in latency sensitive workloads. The other thing is we can also make the log "never gonna full" by making it a bitmap covering any registered ranges, but I don't either know whether it'll be worth it for the effort. Thanks, -- Peter Xu
