[Devel] Re: nsgroup autoremoving

[KAMEZAWA Hiroyuki]

On Sun, 18 Jan 2009 17:32:16 -0600
"Serge E. Hallyn" <serue at us.ibm.com> wrote:

> Quoting Daniel Lezcano (daniel.lezcano at free.fr):
> > Serge E. Hallyn wrote:
> >> Quoting Daniel Lezcano (daniel.lezcano at free.fr):
> >>   
> >>> Hi,
> >>>
> >>> While trying to unshare a namespace with the clone syscall with an  
> >>> inifinite loop, I got an EEXIST.
> >>> That looks weird to have such syscall returning EEXIST ... :)
> >>>
> >>> After investigating, it appears the ns_cgroup creates automatically a 
> >>> control group named with the pid number when we call the clone 
> >>> syscall with a namespace parameter and when the namespace exits, the 
> >>> control group is not automatically removed. So when the pid numbers 
> >>> are recycled we conflict with a previous ns_cgroup name and the clone 
> >>> fails.
> >>>
> >>> IMHO, if the nsgroup is automatically created, it should 
> >>> automatically destroyed, otherwise what will happen to application 
> >>> using the namespaces (eg. mount namespace) wrote before nsgroup 
> >>> appeared ?
> >>>     
> >>
> >> but you can have it automatically destroyed.  I.e. I did the
> >> following:
> >>
> >> mount -t cgroup -o freezer,ns freezer /cgroup
> >> cat > /bin/release_cgroup.sh << EOF
> >> #!/bin/sh
> >> echo "Removing dead cgroup .$*." >> /var/log/cgroup
> >> rmdir /cgroup/$* >> /var/log/cgroup 2>&1
> >> echo "return value was $?" >> /var/log/cgroup
> >> EOF
> >> echo /bin/release_cgroup.sh > /cgroup/release_agent
> >> echo 1 > /cgroup/notify_on_release
> >> chmod ugo+x /bin/release_cgroup.sh
> >> ns_exec -m /bin/sh
> >> ls /cgroup`
> >> 	3581  notify_on_release  release_agent  tasks
> >> exit
> >> ls /cgroup
> >> 	notify_on_release  release_agent  tasks
> >>   
> > Assuming you mount with all the subsystems, this script will destroy the  
> > non-nsgroup too. Each time I create a control group manually, I have to  
> > unset the notify_on_release, right ?
> 
> I assume notify_on_release is per-hierarchy.  So you're just asking
> about manually created cgroups in a hierarchy which has ns mounted,
> right?
> 
> I suppose you could use a naming convention and do some name
> checking in the release_agent to not delete manually created
> ones.
> 
> Would that be too much of a hassle?
> 
> Maybe you're right.  Maybe we should tag auto-created cgroups,
> and auto-remove them.

I think auto-remove is more useful.

>  It's more convenient for me that way...
> Paul, would you have any objections?  Daniel do you have a patch
> written?
> 

Just a notice:
When the memory subsystem is mounted, notify_on_release will not work as you
expected. Because refcnts from pages still exits.

But you will be able to do rmdir() in many case because of pre_destroy()
handler. (so, the directroy is releasable.)
I'd like to fix this. But now, it doesn't work for memory subsys.

Thanks,
-Kame

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