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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, 10 Feb 2015 18:49:53 +0000 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
* Dr. David Alan Gilbert (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?)
As a follow up question; why is 'balloon_page' part of virtio-balloon.c
rather than balloon.c ?
I'm thinking of implementing (3) by putting a queue in front
of balloon_page, but it seems to make more sense to put that
type of thing in shared code (migration shouldn't need to know
it's virtio that's the transport?)
Dave
>
> Thoughts?
>
> Dave
>
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK