On 10 Jun 2026, at 9:17, Michael S. Tsirkin 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.
>
> Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page
> reports to host")
> Signed-off-by: Michael S. Tsirkin <[email protected]>
> Assisted-by: Claude:claude-opus-4-6
> ---
> Changes v1->v2:
> - Document capacity=0 as default in commit log
> - Document that capacity need not be a power of two
> - Drop unnecessary comment about integer division cost
> - Update comment on capacity field: "0 (default) means
> PAGE_REPORTING_CAPACITY"
>
> drivers/virtio/virtio_balloon.c | 5 +----
> include/linux/page_reporting.h | 3 +++
> mm/page_reporting.c | 24 ++++++++++++------------
> 3 files changed, 16 insertions(+), 16 deletions(-)
>
LGTM. With David's concern addressed, feel free to add:
Acked-by: Zi Yan <[email protected]>
--
Best Regards,
Yan, Zi
