CR in namespace

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

Problem[edit]

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[edit]

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. It's a wrapper, use it the same way you use regular criu binary, all command line options are the same.

Dumping processes restored by criu-ns[edit]

Note that if you want to dump process trees that were restored by criu-ns, you need to use criu-ns dump.

Limitations[edit]

  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)