[CRIU] problem dumping some kinds of lxc containers

[Pavel Emelyanov]

On 08/27/2014 03:18 AM, Tycho Andersen wrote:
> Hi all,
> 
> I'm trying to dump an lxc container (created with the ubuntu-cloud
> template). I get:
> 
> (00.563988) Error (files-reg.c:457): Can't link remap to /proc/20/mountinfo: No such file or directory
> 
> /proc/20 doesn't exist, and when this happens there is no pid in the
> container with pid 20. This is a little confusing, though, since
> fill_fdlink() takes a struct fd_parms with a pid in the host pid ns,
> but gives back the path in the container pid ns.
> 
> After a bit of debugging, I found that the process that is causing
> this problem is:
> 
> root     17593  0.3  0.0  26052  1340 ?        S    17:49   0:00 \_ mountall --daemon
> 
> If I try to checkpoint the container after mountall has exited, it all
> works fine.
> 
> Any ideas what is going on here?

Yes. CRIU finds an open file, that cannot be opened by the path kernel provides.
In your case this is because task 20 has died. At the same time stat() reports
that the link count on that file is not 0 (this is due to how proc works), which
in case of disk file would mean, that file "should exist" and we just have to
create some other name for it. This is called "link remap". For disk files CRIU
handles it by creating a hard link on the file. For proc this will obviously not
work, we have to invent something else.

> Thanks,
> 
> Tycho
> _______________________________________________
> CRIU mailing list
> CRIU at openvz.org
> https://lists.openvz.org/mailman/listinfo/criu
>