[CRIU] Podman container migration

[Radostin Stoyanov]

On 09/01/2019 18:53, Adrian Reber wrote:
> On Wed, Jan 09, 2019 at 02:34:31PM +0000, Radostin Stoyanov wrote:
>> On 09/01/2019 14:16, Adrian Reber wrote:
>>> I started to look into migrating containers with Podman and I am
>>> currently stuck at files which are bind-mounted into the container:
>>>
>>> If I checkpoint a container and restore it, it works.
>>>
>>> If I checkpoint a container and transfer the checkpoint to another
>>> container it only works if the previous container is running, as
>>> CRIU fails to mount the file into the container.
>>>
>>> Following is from the checkpointed container:
>>>
>>>         {
>>>             "fstype": 5, 
>>>             "mnt_id": 368, 
>>>             "root_dev": "0:20", 
>>>             "parent_mnt_id": 367, 
>>>             "flags": "0x1000006", 
>>>             "root": "/containers/storage/overlay-containers/43d02fb390d236ec42c5119b6655be44157e4a61103bc827db3b0718feecc898/userdata/run/secrets", 
>>>             "mountpoint": "/run/secrets", 
>>>             "source": "tmpfs", 
>>>             "options": "size=2899748k,nr_inodes=724937,mode=755", 
>>>             "shared_id": 0, 
>>>             "master_id": 0, 
>>>             "sb_flags": "0x0", 
>>>             "ext_key": "/run/secrets"
>>>         }, 
>>>
>>>
>>> The main problem I see is that the storage location of the checkpointed
>>> container is part of "root".
>>>
>>> Or do I have to make sure that the migrated container is using the same
>>> paths as the restored container.
>>>
>>> I get the following error during restore, if I copy the checkpoint to another
>>> container.
>>>
>>> (00.009274)      1: mnt: Start with 0:/tmp/.criu.mntns.d6UW28
>>> (00.009279)      1: mnt: 	Mounting overlay @/tmp/.criu.mntns.d6UW28/12-0000000000/ (0)
>>> (00.009285)      1: mnt: 367:/tmp/.criu.mntns.d6UW28/12-0000000000/ private 1 shared 0 slave 0
>>> (00.009295)      1: mnt: 	Mounting tmpfs @/tmp/.criu.mntns.d6UW28/12-0000000000/run/secrets (0)
>>> (00.009297)      1: mnt: 	Bind /var/run/containers/storage/overlay-containers/43d02fb390d236ec42c5119b6655be44157e4a61103bc827db3b0718feecc898/userdata/run/secrets to /tmp/.criu.mntns.d6UW28/12-0000000000/run/secrets
>>> (00.009326)      1: Error (criu/mount.c:2279): mnt: Can't mount at /tmp/.criu.mntns.d6UW28/12-0000000000/run/secrets: No such file or directory
>>> (00.009331)      1: mnt: Start with 0:/tmp/.criu.mntns.d6UW28
>>> (00.027848) Error (criu/mount.c:3274): mnt: Can't remove the directory /tmp/.criu.mntns.d6UW28: No such file or directory
>>> (00.027860) Error (criu/cr-restore.c:2300): Restoring FAILED.
>>>
>>> The checkpointed container has the ID 43d02fb390d236ec42c5119b6655be44157e4a61103bc827db3b0718feecc898
>>> and the restored container has the ID 5ff14ac246ec6624c9667767532436d6fa5e3abda2b4437dc4b0db2ff1ff661a
>>>
>>> How do I tell CRIU to use another directory for the bind mount?
>> What about "--external mnt[KEY]:VAL" ?
> This is not exported as a runc interface.
In the rpc.proto file we have:

message ext_mount_map {
    required string        key    = 1;
    required string        val    = 2;
};

It is also added in libcriu as "criu_add_ext_mount()".

> And runc already does some magic with the mounts:
>
> func (c *linuxContainer) addCriuDumpMount(req *criurpc.CriuReq, m *configs.Mount) {
>         mountDest := m.Destination
>         if strings.HasPrefix(mountDest, c.config.Rootfs) {
>                 mountDest = mountDest[len(c.config.Rootfs):]
>         }
>
>         extMnt := &criurpc.ExtMountMap{
>                 Key: proto.String(mountDest),
>                 Val: proto.String(mountDest),
>         }
>         req.Opts.ExtMnt = append(req.Opts.ExtMnt, extMnt)
> }
>
> There is also the option to mask paths, not sure if that helps. So
> actually this is more a runc/criu question. As Andrei wrote the code in
> 2015 I am hoping he still remembers how this should be handled (if it
> can be handled at all...).
>
> 		Adrian