[Devel] Re: What can OpenVZ do?

[Alexey Dobriyan]

On Tue, Feb 17, 2009 at 04:40:39PM -0800, Dave Hansen wrote:
> On Wed, 2009-02-18 at 01:32 +0100, Ingo Molnar wrote:
> > > > 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.
> 
> OK, we can definitely do that.  Do you think it is OK to run through a
> set of checks at exec() time to check if the app currently has any
> unsupported things going on?  If we don't directly inherit the parent's
> status, then we need to have *some* time when we check it.

Uncheckpointable is not one-way.

Imagine remap_file_pages(2) is unsupported. Now app uses
remap_file_pages(2), then unmaps interesting VMA. Now app is
checkpointable again.

As for overloading LSM, I think, it would be horrible.
Most hooks are useless, there are config options expanding LSM hooks,
and CPT and LSM are just totally orthogonal.

Instead, just (no offence) get big enough coverage -- run modern and
past distros, run servers packaged with them, and if you can checkpoint
all of this, you're mostly fine.
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