[Devel] Re: What can OpenVZ do?

[Dave Hansen]

On Tue, 2009-02-17 at 23:23 +0100, Ingo Molnar wrote:
> * Dave Hansen <dave at linux.vnet.ibm.com> wrote:
> > On Fri, 2009-02-13 at 11:53 +0100, Ingo Molnar wrote:
> > > In any case, by designing checkpointing to reuse the existing LSM
> > > callbacks, we'd hit multiple birds with the same stone. (One of
> > > which is the constant complaints about the runtime costs of the LSM
> > > callbacks - with checkpointing we get an independent, non-security
> > > user of the facility which is a nice touch.)
> > 
> > There's a fundamental problem with using LSM that I'm seeing 
> > now that I look at using it for file descriptors.  The LSM 
> > hooks are there to say, "No, you can't do this" and abort 
> > whatever kernel operation was going on.  That's good for 
> > detecting when we do something that's "bad" for checkpointing.
> > 
> > *But* it completely falls on its face when we want to find out 
> > when we are doing things that are *good*.  For instance, let's 
> > say that we open a network socket.  The LSM hook sees it and 
> > marks us as uncheckpointable.  What about when we close it?  
> > We've become checkpointable again.  But, there's no LSM hook 
> > for the close side because we don't currently have a need for 
> > it.
> 
> Uncheckpointable should be a one-way flag anyway. We want this 
> to become usable, so uncheckpointable functionality should be as 
> painful as possible, to make sure it's getting fixed ...

Again, as these patches stand, we don't support checkpointing when
non-simple files are opened.  Basically, if a open()/lseek() pair won't
get you back where you were, we don't deal with them.

init does non-checkpointable things.  If the flag is a one-way trip,
we'll never be able to checkpoint because we'll always inherit init's !
checkpointable flag.  

To fix this, we could start working on making sure we can checkpoint
init, but that's practically worthless.

-- Dave

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