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Re: [Qemu-devel] balloon vs postcopy migrate
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] balloon vs postcopy migrate |
Date: |
Tue, 3 Feb 2015 17:38:16 +0000 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
* Pankaj Gupta (address@hidden) 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 / address@hidden / Manchester, UK
> >
> >
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK