On Sun, Jul 05, 2026 at 01:16:35AM -0700, Andrew Morton wrote:
> On Sun, 5 Jul 2026 02:25:13 -0400 "Michael S. Tsirkin" <[email protected]>
> wrote:
>
> > At the moment, if a virtio balloon device has a page reporting vq but
> > its size is < PAGE_REPORTING_CAPACITY (32), the balloon driver fails
> > probe.
> >
> > But, there's no way for host to know this value, so it can easily
> > create a smaller vq and suddenly adding the reporting capability
> > to the device makes all of the driver fail. Not pretty.
> >
> > Add a capacity field to page_reporting_dev_info so drivers can
> > control the maximum number of pages per report batch.
> >
> > In virtio-balloon, set the capacity to the reporting virtqueue size,
> > letting page_reporting adapt to whatever the device provides.
> >
> > Capacity need not be a power of two. Code previously called out
> > division by PAGE_REPORTING_CAPACITY as cheap since it was a power
> > of 2, but no performance difference was observed with non-power-of-2
> > values.
> >
> > If capacity is 0 or exceeds PAGE_REPORTING_CAPACITY, it defaults
> > to PAGE_REPORTING_CAPACITY. The 0 check and the clamping is done in
> > page_reporting_register(), before the reporting work is scheduled,
> > so we never get division by 0.
>
> Thanks. What's the priority here? Should we fix 7.2? Earlier?
>
> It seems that Sashiko has found a pre-existing issue, a hard-to-hit
> error path thing:
>
>
> https://sashiko.dev/#/patchset/444c24cf39f3f3620fc90ef4695bd6b0979f4c4b.1783232420.git....@redhat.com
>
Ugh. Yes but it is a minor symptom actually(
This is the root cause:
virtio_device_ready(vdev);
if (towards_target(vb))
virtballoon_changed(vdev);
return 0;
DRIVER_OK set almost the last thing. But, e.g.:
if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
struct scatterlist sg;
unsigned int num_stats;
vb->stats_vq = vqs[VIRTIO_BALLOON_VQ_STATS];
/*
* Prime this virtqueue with one buffer so the hypervisor can
* use it to signal us later (it can't be broken yet!).
*/
num_stats = update_balloon_stats(vb);
sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
err = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
GFP_KERNEL);
if (err) {
dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
__func__);
return err;
}
virtqueue_kick(vb->stats_vq);
}
this happens before DRIVER_OK and it's quite out of spec.
All of balloon initialization needs to be rethought and fixed.
Maybe.
After coffee.
--
MST
