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

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 5 Jul 2026 02:25:13 -0400 "Michael S. Tsirkin" <mst@redhat.com> 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@redhat.com