24-28 August 2020
US/Pacific timezone

CRIU mounts migration: problems and solutions

24 Aug 2020, 07:25
Microconference1/Virtual-Room (LPC 2020)


LPC 2020

Containers and Checkpoint/Restore MC Containers and Checkpoint/Restore MC


Pavel Tikhomirov


OpenVZ and Virtuozzo containers use CRIU as the core technology for
container migration in production. And Virtuozzo containers are slightly
different thing to what most people would imagine containers today. They are
"system containers" which is the one with full systemd inside, the one you
would enter via ssh, the one which is an analogy to a virtual machine where the
user gets root access inside and can do almost everything like on the hardware
node with Linux.

This difference between application and system containers brings a lot of
complex problems when it comes to container migration of the system
containers. Lets consider the mounts problem. The user inside a container
can explicitly or implicitly (by systemd, docker or some other means)
create multiple different mount namespaces and mounts in them. And if we
migrate the container, the user inside does not expect their mounts to
change. So we need to checkpoint and restore them.

In this talk I would share main problems I've faced when I tried to improve
the correctness of our current mount restore algorithm in CRIU and I would
show new "mounts-v2" algorithm which tries to cover much more cases than
the previous one. To achieve this we need at least one kernel patch [1] and
maybe more to come.

I would like to restart the discussion on bind mounts across namespaces at
the point it had stopped a while ago. I hope we can reach a consensus about
the kernel modifications required to solve the problem of
checkpoint/restore of complex mounts. And I really hope for some useful
advice on how to further improve the new algorithm.

[1] https://lore.kernel.org/lkml/1485214628-23812-1-git-send-email-avagin@openvz.org/

Here are links to mounts-v2 implementation in Virtuozzo criu:
- Main part: https://src.openvz.org/projects/OVZ/repos/criu/commits?until=v3.12.3.12
- Delayed proc part: https://src.openvz.org/projects/OVZ/repos/criu/commits?until=v3.12.5.13

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