Re: [PATCH 2/2] virtio_balloon: quiesce balloon work before device shutdown

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 22, 2026 at 04:38:54PM +0200, David Hildenbrand (Arm) wrote:
> On 6/22/26 15:37, Denis V. Lunev wrote:
> > Commit 8bd2fa086a04 ("virtio: break and reset virtio devices on
> > device_shutdown()") added a generic virtio bus .shutdown handler that
> > breaks and resets every virtio device during device_shutdown(), i.e. on
> > reboot and kexec.
> > 
> > virtio_balloon provides no .shutdown of its own, so that generic path
> > runs while the balloon's asynchronous work is still armed. Once the
> > device has been broken, virtqueue_add_inbuf() in
> > virtballoon_free_page_report() returns -EIO and trips its
> > WARN_ON_ONCE(). On a kernel booted with panic_on_warn that turns an
> > ordinary reboot, for example a kexec based upgrade, into a fatal panic
> > in the middle of device_shutdown(), so the machine never reaches the
> > new kernel.
> > 
> > Relaxing that single WARN_ON_ONCE() would only hide the symptom: the
> > inflate/deflate and OOM paths do not warn, they call
> > wait_event(vb->acked, ...) and would instead block forever on a broken
> > queue that can no longer complete. The device has to be quiesced, not
> > just kept quiet.
> 
> Ah, so
> 
> 	/* We should always be able to add one buffer to an empty queue. */
> 	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> 
> is not actually correct.

Yes - we can't if the device is completely gone)

> Yeah, quiescing sounds cleaner, although I am thinking whether we should also
> warn if virtqueue_add_outbuf() fails, similar to what we do in
> virtballoon_free_page_report().
> 
> > 
> > Add a .shutdown handler that quiesces the balloon via the shared
> > virtballoon_quiesce() helper while the device is still alive, and only
> > then breaks and resets it. The break and reset are repeated here rather
> > than reused from virtio_dev_shutdown(): drv->shutdown replaces the
> > generic handler rather than augmenting it, so that drivers such as
> > virtio-gpu can opt out of the reset. Unlike virtballoon_remove() the
> > balloon workqueue is not destroyed, as shutdown does not free the
> > device and cancel_work_sync() together with stop_update already prevent
> > any further work from being queued.
> > 
> > Fixes: 8bd2fa086a04 ("virtio: break and reset virtio devices on device_shutdown()")
> > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > ---
> >  drivers/virtio/virtio_balloon.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 5b02d9191ac6..e35ada767b4b 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -1137,6 +1137,15 @@ static void virtballoon_remove(struct virtio_device *vdev)
> >  	kfree(vb);
> >  }
> >  
> > +static void virtballoon_shutdown(struct virtio_device *vdev)
> > +{
> > +	virtballoon_quiesce(vdev->priv);
> > +
> > +	virtio_break_device(vdev);
> > +	virtio_synchronize_cbs(vdev);
> > +	vdev->config->reset(vdev);
> 
> I guess it would be good if we wouldn't have to copy what the default handler
> does, but could instead just have it in a reusable core function?
> 
> > +}
> > +
> 
> -- 
> Cheers,
> 
> David