Re: [Qemu-devel] [PATCH] hw/virtio/balloon: Fixes for different host page sizes

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

* Thomas Huth (thuth@redhat.com) wrote:
> On 14.04.2016 13:47, Dr. David Alan Gilbert wrote:
> > * Thomas Huth (thuth@redhat.com) wrote:
> > 
> >> That would mean a regression compared to what we have today. Currently,
> >> the ballooning is working OK for 64k guests on a 64k ppc host - rather
> >> by chance than on purpose, but it's working. The guest is always sending
> >> all the 4k fragments of a 64k page, and QEMU is trying to call madvise()
> >> for every one of them, but the kernel is ignoring madvise() on
> >> non-64k-aligned addresses, so we end up with a situation where the
> >> madvise() frees a whole 64k page which is also declared as free by the
> >> guest.
> > 
> > I wouldn't worry about migrating your fragmenet map; but I wonder if it
> > needs to be that complex - does the guest normally do something more sane
> > like do the 4k pages in order and so you've just got to track the last
> > page it tried rather than having a full map?
> 
> That's maybe a little bit easier and might work for well-known Linux
> guests, but IMHO it's even more a hack than my approach: If the Linux
> driver one day is switched to send the pages in the opposite order, or
> if somebody tries to run a non-wellknown (i.e. non-Linux) guest, this
> does not work at all anymore.

True.

> > A side question is whether the behaviour that's seen by virtio_ballon_handle_output
> > is always actually the full 64k page;  it calls balloon_page once
> > for each message/element - but if all of those elements add back up to the full
> > page, perhaps it makes more sense to reassemble it there?
> 
> That might work for 64k page size guests ... but for 4k guests, I think
> you'll have a hard time to reassemble a page there more easily than with
> my current approach. Or do you have a clever algorithm in mind that
> could do the job well there?

No, i didn't; I just have an ulterior motive which is trying to
do as few madvise's as possible, and while virtio_balloon_handle_output sees
potentially quite a few requests at once, balloon_page is stuck down
there at the bottom without any idea of whether there are any more coming.

Dave

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