Re: How much of a mess does OpenVZ make? ;) Was: What can OpenVZ do?

Quoting Cedric Le Goater (legoater@free.fr):

quoted

No, what you're suggesting does not suffice.

probably. I'm still trying to understand what you mean below :)

Man, I hate these hierarchicals pid_ns. one level would have been enough, 
just one vpid attribute in 'struct pid*'

Well I don't mind - temporarily - saying that nested pid namespaces
are not checkpointable.  It's just that if we're going to need a new
syscall anyway, then why not go ahead and address the whole problem?
It's not hugely more complicated, and seems worth it.

quoted

Call
(5591,3,1) the task knows as 5591 in the init_pid_ns, 3 in a child pid
ns, and 1 in grandchild pid_ns created from there.  Now assume we are
checkpointing tasks T1=(5592,1), and T2=(5594,3,1).

We don't care about the first number in the tuples, so they will be
random numbers after the recreate. 

yes.

quoted

But we do care about the second numbers.  

yes very much and we need a way set these numbers in alloc_pid()

quoted

But specifying CLONE_NEWPID while recreating the process tree
in userspace does not allow you to specify the 3 in (5594,3,1).

I haven't looked closely at hierarchical pid namespaces but as we're
using a an array of pid indexed but the pidns level, i don't see why 
it shouldn't be possible. you might be right.

anyway, I think that some CLONE_NEW* should be forbidden. Daniel should
send soon a little patch for the ns_cgroup restricting the clone flags
being used in a container.

Uh, that feels a bit over the top.  We want to make this
uncheckpointable (if it remains so), not prevent the whole action.
After all I may be running a container which I don't plan on ever
checkpointing, and inside that container running a job which i do
want to migrate.

So depending on if we're doing the Dave or the rest-of-the-world
way :), we either clear_bit(pidns->may_checkpoint) on the parent
pid_ns when a child is created, or we walk every task being
checkpointed and make sure they each are in the same pid_ns.  Doesn't
that suffice?

-serge

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