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Line 37: − * The [[Lazy migration]] use-case might be racy because there is no means to synchronize between pending forks, remote pages transfers and page faults. Line 45:
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* The <code>restore</code> action accepts yet another API switch: option <code>--lazy-pages</code>. In this mode, <code>restore</code> skips injection of lazy pages into the processes address space, but rather registers lazy memory areas with userfaultfd.
* The <code>restore</code> action accepts yet another API switch: option <code>--lazy-pages</code>. In this mode, <code>restore</code> skips injection of lazy pages into the processes address space, but rather registers lazy memory areas with userfaultfd.
* The lazy pages are completely handled by dedicated <code>lazy-pages</code> daemon. The daemon recieves userfault file descriptors from <code>restore</code> via UNIX socket. The userfault file descriptors allow reception of page-fault and other events and resolution of these events by the daemon.
* The lazy pages are completely handled by dedicated <code>lazy-pages</code> daemon. The daemon receives userfault file descriptors from <code>restore</code> via UNIX socket. The userfault file descriptors allow reception of page-fault and other events and resolution of these events by the daemon.
* For the migration case, the <code>dump</code> action also accepts API switch: option <code>--lazy-pages</code>. When this option is used, the <code>dump</code> keeps the memory pages and allows the <code>lazy-pages</code> daemon to request these pages via TCP connection.
* For the migration case, the <code>dump</code> action also accepts API switch: option <code>--lazy-pages</code>. When this option is used, the <code>dump</code> keeps the memory pages and allows the <code>lazy-pages</code> daemon to request these pages via TCP connection.
Tasks after restore have lazy VMAs registered with userfaultfd, the fd itself is sent before resume to <code>lazy-pages</code> daemon and closed. The daemon monitors the UFFD events and repopulates the tasks address space. The <code>lazy-pages</code> daemon can get pages either from images (both local and remote) or directly from the remote side <code>dump</code>.
Tasks after restore have lazy VMAs registered with userfaultfd, the fd itself is sent before resume to <code>lazy-pages</code> daemon and closed. The daemon monitors the UFFD events and repopulates the tasks address space. The <code>lazy-pages</code> daemon can get pages either from images (both local and remote) or directly from the remote side <code>dump</code>.
When the restored task accesses a missing memory page, it causes a page fault. The <copde>lazy-pages</code> daemon receives the page fault notification and resolves it by populating the faulting task memory. If there were no page faults for some time, the daemon copies the task's remaining memory pages in the background.
When the restored task accesses a missing memory page, it causes a page fault. The <code>lazy-pages</code> daemon receives the page fault notification and resolves it by populating the faulting task memory. If there were no page faults for some time, the daemon copies the task's remaining memory pages in the background.
==== Local images ====
==== Local images ====
* The [[page server]] is run on the remote side with <code>--lazy-pages</code> option.
* The [[page server]] is run on the remote side with <code>--lazy-pages</code> option.
* The lazy-pages daemon connects to the remote [[page server]] with <code>--page-server</code> option. The <code>--address</code> and <code>--port</code> options allow setting of IP addrees and port of the listening [[page server]].
* The lazy-pages daemon connects to the remote [[page server]] with <code>--page-server</code> option. The <code>--address</code> and <code>--port</code> options allow setting of IP address and port of the listening [[page server]].
* Current protocol allows the lazy-pages daemon to request several continous pages.
* Current protocol allows the lazy-pages daemon to request several continuous pages.
==== Migration ====
==== Migration ====
* Currently only MAP_PRIVATE | MAP_ANONYMOUS is supported. Newer kernels (4.11+) allow userfaultfd for hugetlbfs and shared memory, yet to be implemented in CRIU.
* Currently only MAP_PRIVATE | MAP_ANONYMOUS is supported. Newer kernels (4.11+) allow userfaultfd for hugetlbfs and shared memory, yet to be implemented in CRIU.
* Userfault is known not to map one page into two places. Thus -- COW-ed pages will get COW-ed.
* Userfault is known not to map one page into two places. Thus -- COW-ed pages will get COW-ed.
== See also ==
== See also ==
[[Category:Memory]]
[[Category:Memory]]
[[Category:Plans]]
[[Category:New features]]
[[Category:Under the hood]]
[[Category:Under the hood]]