[CRIU] Podman container migration

[Adrian Reber]

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. 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