Re: [Qemu-devel] balloon vs postcopy migrate

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Pankaj Gupta (pagupta@redhat.com) wrote:
> 
> > Hi,
> >   Andrea pointed out there is a risk that a guest inflating its
> > balloon during a postcopy migrate could cause us problems, and
> > I wanted to see what the best way of avoiding the problem was.
> > 
> > Guests inflating there balloon cause an madvise(MADV_DONTNEED) on
> > the host, marking pages as not present, that will potentially trigger
> > a userfault, that we are using in postcopy to detect pages that need
> > to be fetched from the source.
> > 
> > In theory, at the moment guests *should* only ask for a balloon
> > inflation if they've been asked to do so by the host; however there
> > are no guards for that, and it's been suggested giving the
> > guest more freedom might be a good idea anyway.
> > 
> > My alternatives seem to be:
> >    1) Stop servicing the message queue from the guest so
> >      that we just don't notice the inflate messages until
> >      afterwards.  (Easy for Qemu, not sure how the guests
> >      will like an unserviced queue).
> > 
> >    2) I could keep servicing the queue and ignore the messages
> >      (Easy for everyone, not very nice in actual used memory -
> >       does it cause any long term problems other than that?)
> > 
> >    3) I could keep servicing the queue but put the messages
> >      in a list somewhere that replay after migrate has finished.
> >      (That list sounds bounded only in a very large way?)
> > 
> > Thoughts?
> 
> Can we have some global flag somewhere when Post copy is ON/active.
> And we can ignore or defer only inflate/ballon messages/commands while 
> servicing the commands with some warnings.
> 
> Just my thought on logic. Not sure if I am missing some background here.

Oh yes, the global flag is the easy part; the only question is what
the best thing to do is when it's set.

Dave

> 
> > 
> > Dave
> > 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> > 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK