CR in namespace

This page describes the criu-ns script that has appeared in criu-dev branch after 2.5 and is aimed at 2.6.

Problem

When restoring a process tree without namespaces (i.e. into namespaces in which criu lives) one may frequently hit a "pid mismatch" problem. It happens when a process CRIU tries to restore has a PID which is already owned by some other process living in this pid namespace. There's no way one can restore a process tree in this namespace, so the only solution is to unshare the new pid namespace and restore the tree into it.

When doing so we have two problems:

1. In the new pid namespace, someone has to become the "init" process
2. In the new pid namespace, we'd need to have new /proc mount which implies unsharing the mount namespace as well

Even if one solves the above problems, there will appear the third one -- when trying to dump the restore process tree again criu will likely refuse to dump the mount namespace, as it will be a complete clone of the original one and thus would likely contain many many unsupported mounts such as real devices, securityfs, trace/debug-fs etc. The latter can still be solved by nsenter-ing the process' pid and mount namespaces and doing the dump.

Solution

All of the above -- the need to unshare stuff, getting pseudo-init, re-mounting /proc on dump and setns-ing on restore -- is solved by the scripts/criu-ns script. Use it the same way you use regular criu binary, all command line options are the same.

Limitations

1. Shell jobs are not supported
2. The swrk mode is not supported
3. Only the dump and restore are supported (incremental dumps are not possible)