[Devel] Re: What can OpenVZ do?

[Ingo Molnar]

* 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 ...

> We have a couple of options:
> 
> We can let uncheckpointable actions behave like security 
> violations and just abort the kernel calls.  The problem with 
> this is that it makes it difficult to do *anything* unless 
> your application is 100% supported. Pretty inconvenient, 
> especially at first.  Might be useful later on though.

It still beats "no checkpointing support at all in the upstream 
kernel", by a wide merging. If an app fails, the more reasons to 
bring checkpointing support up to production quality? We dont 
want to make the 'interim' state _too_ convenient, because it 
will quickly turn into the status quo.

Really, the LSM approach seems to be the right approach here. It 
keeps maintenance costs very low - there's no widespread 
BKL-style flaggery.

	Ingo
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