[CRIU] problem dumping some kinds of lxc containers

[Tycho Andersen]

Hi Pavel,

On Wed, Aug 27, 2014 at 12:35:07PM +0400, Pavel Emelyanov wrote:
> 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 for the explanation. Any ideas on what the proper solution is?

Tycho

> > Thanks,
> > 
> > Tycho
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>