Re: [Qemu-devel] [PATCH] set VIRTIO_BALLOON_F_MUST_TELL_HOST unconditionally

[Anthony Liguori]

On 04/15/2011 11:36 AM, Dave Hansen wrote:
> On Fri, 2011-04-15 at 11:21 -0500, Anthony Liguori wrote:
> > > This patch makes the "tell host first" logic the only case.  This
> > > should make everybody happy, and reduce the amount of untested or
> > > untestable code in the kernel.
> > It doesn't make me happy.
> Darn.
>
> > Why would we do this in QEMU?  This prevents the guest from doing
> > ballooning reclaim during OOM.
> What the heck is "ballooning reclaim"?  Could you elaborate a bit on how
> this happens?  I think I'm missing some subtlety here.

If you're in OOM and you need memory, you can't ask the host for more 
and wait for a response.  You have to reclaim it immediately.

See the s390 balloon driver for an example of this.

> I think 'tell host first' is the only sane way to do it.  Look at the
> 'tell host second code':
>
>         release_pages_by_pfn(); // let other kernel users at the pages
>         tell_host(); // tell the hypervisor they're used again

> At the point we've started using the pages again, we haven't *told* the
> host that we're using them.  I think that's potentially a problem.  Is
> qemu somehow cool with the guest touching pages that are supposed to be
> in the balloon and unusable?

Yes, of course it is.  All ballooning does is madvise(MADV_DONTNEED).  A 
guest can reclaim the memory as soon as it wants it to.

Regards,

Anthony Liguori

> -- Dave