[CRIU] rewriting images via criu (or crit?)

[Tycho Andersen]

Hi all,

One of the problems we talked about at Plumbers was how to re-write cgroups. For
example, if you migrate a container from a node with 4 cores (and thus a
cpuset.cpus of 0-3), to a node with two cores (cpuset = 0-1), a write of 0-3
into the new host's cpuset.cpus will fail, and the restore will fail.

Thus, we need some way to re-write these things (and images in general) so that
we can handle this case and others. One option is to have a small syntax to
rewrite things, and add support to the new crit tool either in C or in python
depending on speed concerns to rewrite the images. Stealing some inspiration
from xpath, below is my proposal for a language that does this rewriting.

In our example above, we want to write cpuset.cpus everywhere from 0-3 to 0-1:

cgroup_entry.controllers["cpuset" in cnames]//properties[name="cpuset.cpus"] set value=0-1

That is, first, match the buffer message type "cgroup_entry" with any
controllers who have "cpuset" in them (the 'in' here would only work for
'repeated' message types), "//" says "recursively match any object with...",
properties[name="cpuset.cpus"] says match any object in properties where the
name is "0-3". Finally, set (get, delete) says what to do, and value says 'set
the value member of each matching object to "0-3"'.

To do something a bit simpler, like setting a tasks exit code, you could do:

core_entry.tc set exit_code=1

That is, match all core entries' (there is only one in each image) tc object
without constraints, and set the exit_code value to 1.

Some images have binary data in them, which we can just ignore for now.

The motivation for having a language (vs. just exposing the .proto definitions
and allowing users to use the images directly) is that we don't want to force
users to link against libprotobuf.

Does this sound reasonable? Anyone have any ideas on how to do this in a more
succinct (but still flexible :) fashion?

Thanks,

Tycho