Re: The issues for agreeing on a virtualization/namespaces implementation.

[Hubertus Franke]

Eric W. Biederman wrote:
> Hubertus Franke <frankeh@xxxxxxxxxxxxxx> writes:
>
>
> > Eric W. Biederman wrote:

> > > 2) What is the syscall interface to create these namespaces?
> > >   - Do we add clone flags?       (Plan 9 style)
> >
> > Like that approach .. flexible .. particular when one has well specified
> > namespaces.
> >
> >
> > >   - Do we add a syscall (similar to setsid) per namespace?
> > >     (Traditional unix style)?
> >
> > Where does that approach end .. what's wrong with doing it at clone() time ?
> > Mainly the naming issue. Just providing a flag does not give me name.
>
>
> It really is a fairly even toss up.  The usual argument for doing it
> this way is that you will get a endless stream of arguments added to
> fork+exec other wise.  Look of posix_spawn or the windows version if
> you want an example.  Bits to clone are skirting the edge of a slippery 
> slope.

So it seems the clone( flags ) is a reasonable approach to create new
namespaces. Question is what is the initial state of each namespace?
In pidspace we know we should be creating an empty pidmap !
In network, someone suggested creating a loopback device
In uts, create "localhost"
Are there examples where we rather inherit ?  Filesystem ?
Can we iterate the assumption for each subsystem what people thing is right?

IMHO, there is only a need to refer to a namespace from the global context.
Since one will be moving into a new container, but getting out of one
could be prohibitive (e.g. after migration)
It does not make sense therefore to know the name of a namespace in
a different container.

The example you used below by using the pid comes natural, because
that already limits visibility.

I am still struggling with why we need new sys_calls.
sys_calls already exist for changing certain system parameters (e.g. utsname )
so to me it boils down to identifying a proper initial state when the
namespace is created.

> > > 3) How do we refer to namespaces and containers when we are not members?
> > >   - Do we refer to them indirectly by processes or other objects that
> > >     we can see and are members?
> > >   - Do we assign some kind of unique id to the containers?
> >
> > In containers I simply created an explicite name, which ofcourse colides with
> > the
> > clone() approach ..
> > One possibility is to allow associating a name with a namespace.
> > For instance
> > int set_namespace_name( long flags, const char *name ) /* the once we are using
> > in clone */
> > {
> > 	if (!flag)
> > 		set name of container associated with current.
> > 	if (flag())
> > 		set the name if only one container is associated with the
> > namespace(s)
> > 		identified .. or some similar rule
> > }
>
>
> What I have done which seems easier than creating new names is to refer
> to the process which has the namespace I want to manipulate.

Is then the idea to only allow the container->init to manipulate
or is there need to allow other priviliged processes to perform namespace
manipulation?
Also after thinking about it.. why is there a need to have an external name
for a namespace ?

> > > 6) How do we do all of this efficiently without a noticeable impact on
> > >   performance?
> > >   - I have already heard concerns that I might be introducing cache
> > >     line bounces and thus increasing tasklist_lock hold time.
> > >     Which on big way systems can be a problem.
> >
> > Possible to split the lock up now.. one for each pidspace ?
>
>
> At the moment it is worth thinking about.  If the problem isn't
> so bad that people aren't actively working on it we don't have to
> solve the problem for a little while, just be aware of it.

Agree, just need to be sure we can split it up. But you already keep
a task list per pid-namespace, so there should be no problem IMHO.
If so let's do it now and take it of the table it its as simple as

task_list_lock ::= pspace->task_list_lock

> > > 7) How do we allow a process inside a container to create containers
> > >   for it's children?
> > >   - In general this is trivial but there are a few ugly issues
> > >     here.
> >
> > Speaking of pids only here ...
> > Does it matter, you just hang all those containers hang of init.
> > What ever hierarchy they form is external ...
>
>
> In general it is simple.  For resource accounting, and for naming so
> you can migrate a container with a nested container it is a question
> you need to be slightly careful with.

Absolutely, that's why it is useful to have an "external" idea of how
containers are constructed of basic namespaces==subsystems.
The it "simply" becomes a policy. E.g. one can not migrate a container
that has shared subsystems.
Resource accounting I agree, that might required active aggregation
at request time.

-- Hubertus

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