Re: [PATCH v3] mm: page_reporting: allow driver to set batch capacity

[Michael S. Tsirkin]

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" <mst@xxxxxxxxxx> 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.mst@xxxxxxxxxx
> 

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