[Devel] [RFC][PATCH] Add child reaper to struct pspace

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> "Serge E. Hallyn" <serue at us.ibm.com> writes:
> 
> > Quoting Eric W. Biederman (ebiederm at xmission.com):
> >> Yes.  We all so need something like that list to support kill -1.
> >> Although walking the list of all processes may be sufficient for a first
> >> pass.
> >> 
> >> The real trick is handing nested pid namespaces, properly.
> >
> > Not if, as you've suggested in the past, pid_ns 5 has valid pids in its
> > own pid_ns for every process in pid_namespaces nested under it.
> >
> > It should be simple to implement, should not impact the non-container
> > cases, and should only start to impact performance as the nesting gets
> > deep, which AFAIK we all believe won't happen (max nesting of 2 AFAICS,
> > one checkpointable application container under one vserver-thingie)
> >
> > And it makes kill -1 trivial, as in pid_ns 5 we just kill all processes
> > in pid_ns 5, without worrying about finding the ones in it's decendent
> > pid namespaces.
> 
> If you do it correctly I agree.  But you have to be very careful where
> you put the list.  
> 
> My point being not that we can't get this correct with simple code, but
> that it is easy to get it wrong.

True.

I should think the list has to go into the struct pid, not the
task_struct.  There is one struct pid per (pid_ns, pid), so we can keep
just one simple list to walk the pid_ns processes.  If we were to put it
in the task_struct, well there's really no clean way to go about it then :)

thanks,
-serge
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