Hello, I'm working on integration of userfaultfd into CRIU. Currently we can perform lazy restore and post-copy migration with the help of userfaultfd, but there are some limitations because of incomplete in-kernel support for non-cooperative mode of userfaultfd. I'd like to particpate in userfaultfd-WP discussion suggested by Andrea Acangeli [1]. Besides, I would like to broaden userfaultfd discussion so it will also cover the following topics: * Non-cooperative userfaultfd APIs for checkpoint/restore Checkpoint/restore of an application that uses userfaultfd will require additions to the userfaultfd API. The new APIs are needed to allow saving parts of in-kernel state of userfaultfd during checkpoint and then recreating this state during restore. * Userfaultfd and COW-sharing. If we have two tasks that fork()-ed from each other and we try to lazily restore a page that is still COW-ed between them, the uffd API doesn't give us anything to do it. So we effectively break COW on lazy restore. * Userfaultfd "nesting" [2] CRIU uses soft-dirty to track memory changes. We would like to switch to userfaultfd-WP once it gets merged. If the process for which we are tracking memory changes uses userfaultfd, we would need some notion of uffd "nesting", so that the same memory region could be monitored by different userfault file descriptors. Even more interesting case is tracking memory changes of two different processes: one process that has memory regions monitored by uffd and another one that owns the non-cooperative userfault file descriptor to monitor the first process. The userfaultfd "nesting" is also required for lazy restore scenario so that CRIU will be able to use userfaultfd for memory ranges that the restored application is already managing with userfaultfd. [1] http://www.spinics.net/lists/linux-mm/msg119866.html [2] https://www.spinics.net/lists/linux-mm/msg112500.html -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
