[Devel] Re: What can OpenVZ do?

[Ingo Molnar]

* Dave Hansen <dave at linux.vnet.ibm.com> wrote:

> 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.

i mean, it should be per process (per app) one-way flag of 
course. If the app does something unsupported, it gets 
non-checkpointable and that's it.

	Ingo
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