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

[Dave Hansen <dave@linux.vnet.ibm.com>]

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.

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?

-- Dave