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

[Dave Hansen]

On Fri, 2011-04-15 at 12:17 -0500, Anthony Liguori wrote:
> On 04/15/2011 11:36 AM, Dave Hansen wrote:
> >> 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.

Why not?  The call in to the notifier chain the s390 case is
synchronous.  The OOM only affects one task at a time and won't proceed
elsewhere while this is going on.  

> See the s390 balloon driver for an example of this.

I could buy that in an OOM situation, but that's not what virtio-balloon
is doing.  

> > 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.

That's not true on all hypervisors, but as long as it's a guarantee in
qemu, I guess we're OK here.  Let's hope nobody else ever starts to use
virtio-balloon.

Why do we even _tell_ qemu, though?  The MADV_WILLNEED is nice, but far
from being necessary.  We could just skip the entire notification in OOM
situations.

-- Dave