[Devel] Re: cryo and mm->arg_start

[Serge E. Hallyn]

Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> Serge E. Hallyn [serue at us.ibm.com] wrote:
> | Quoting Matt Helsley (matthltc at us.ibm.com):
> | > 
> | > On Fri, 2008-07-11 at 09:38 -0700, Dave Hansen wrote:
> | > > On Fri, 2008-07-11 at 08:13 -0500, Serge E. Hallyn wrote:
> | > > > 
> | > > > One thing we could do here is to start extending the cryo approach
> | > > > with Eric's checkpoint-as-a-coredump (caac?).  We generate the
> | > > > tiniest of coredumps which, at first, contains nothing but
> | > > > mm->arg_start and maybe a process id.  It would be simplest if
> | > > > it also contained a filename for the real executable,
> | > > 
> | > > The exec model sounds reasonable to me.
> | > >
> | > > But, I think the filename of the exe is going to have to be in the
> | > > checkpoint *already*.  It is mapped by at least one of the VMAs, and
> | > > will probably be dumped as a normal file-backed area.  
> | > 
> | > 	Yes, the file that backed the exec will be there. Note that thanks to
> | > "stacking" filesystems the path to the file backing the exe is not
> | > _always_ going to be the same as the path to the file which userspace
> | > exec'd in the first place. You can see this by comparing
> | > the /proc/<pid>/exe symlink with the file backing the VMA.
> | > 
> | > 	This is important to any program which checks the /proc/self/exe
> | > symlink to find out where it's installed (Java does this, for example).
> | > I think it's possible to do this with a binfmt -- it's just one more
> | > detail to remember.
> | > 
> | > Cheers,
> | > 	-Matt
> | 
> | Let's say that before starting my checkpointable job, I did
> | 
> | 	mount -t ecryptfs /home/hallyn /home/hallyn
> | 
> | Now if the checkpointable job is /home/hallyn/somelongjob, then I think
> | it's fair to say that restart can fail if /home/hallyn at the restart
> | machine isn't ecryptfs-mounted.
> | 
> | In that case, would you still think there is a problem?
> | 
> | On the other hand, if the checkpointable job did the ecryptfs mount
> | itself, then it would be expected that at restart the ecryptfs mount
> | would be remounted.  How that would be done I have no idea offhand.
> 
> Hmm, wonder if the new /proc/pid/mountinfo with its mount-ids would
> enable us to identify the filesystems that a given process expects.

Interesting point.  Yes, it *should*, that's sort of the idea.  I don't
remember whether some of the limitations in terms of hiding mount-ids
from other namespaces were implemented or not, if so I suspect they
could be a problem.

> Which brings up another question. If two processes in the same container
> have different mount namespaces and mount points, we would need to
> reestablish the mounts during restart right ?

Yes.

-serge
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