Hi David,
On 12/10/25 12:09 AM, David Hildenbrand (Red Hat) wrote:
> On 12/9/25 22:23, Dongli Zhang wrote:
>> Do not set vb->pr_dev_info.report unconditionally if
>> VIRTIO_BALLOON_F_REPORTING is not available.
>
> Can you share with us why you think that should be done? Please document the
> "why" and not only the "what".
>
> Without VIRTIO_BALLOON_F_REPORTING, we'll never call
> page_reporting_register(),
> so it will never be used.
>
> But the compiler cannot optimize it out. It only happens during driver
> loading,
> so I am not sure it is worth the churn?
When I was reading about the free-page reporting feature in virtio-balloon, I
was confused as to why pr_dev_info.report was always configured unconditionally.
Later, I looked at the implementation in the Hyper-V balloon driver and noticed
that it even resets pr_dev_info.report back to NULL if page_reporting_register()
fails (see line 1669).
1651 static void enable_page_reporting(void)
1652 {
1653 int ret;
1654
1655 if (!hv_query_ext_cap(HV_EXT_CAPABILITY_MEMORY_COLD_DISCARD_HINT))
{
1656 pr_debug("Cold memory discard hint not supported by
Hyper-V\n");
1657 return;
1658 }
1659
1660 BUILD_BUG_ON(PAGE_REPORTING_CAPACITY >
HV_MEMORY_HINT_MAX_GPA_PAGE_RANGES);
1661 dm_device.pr_dev_info.report = hv_free_page_report;
1662 /*
1663 * We let the page_reporting_order parameter decide the order
1664 * in the page_reporting code
1665 */
1666 dm_device.pr_dev_info.order = 0;
1667 ret = page_reporting_register(&dm_device.pr_dev_info);
1668 if (ret < 0) {
1669 dm_device.pr_dev_info.report = NULL;
1670 pr_err("Failed to enable cold memory discard: %d\n", ret);
1671 } else {
1672 pr_info("Cold memory discard hint enabled with order %d\n",
1673 page_reporting_order);
1674 }
1675 }
That's why I'd like to move the configuration of pr_dev_info.report inside the
if statement.
It's a purely non-functional change, intended only to make the initialization
look cleaner.
Apologies - I should have mentioned that this is a non-functional change in the
commit message.
Thank you very much!
Dongli Zhang
>
>>
>> Signed-off-by: Dongli Zhang <[email protected]>
>> ---
>> drivers/virtio/virtio_balloon.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/virtio/virtio_balloon.c
>> b/drivers/virtio/virtio_balloon.c
>> index 74fe59f5a78c..0c39f2415324 100644
>> --- a/drivers/virtio/virtio_balloon.c
>> +++ b/drivers/virtio/virtio_balloon.c
>> @@ -1034,7 +1034,6 @@ static int virtballoon_probe(struct virtio_device
>> *vdev)
>> poison_val, &poison_val);
>> }
>> - vb->pr_dev_info.report = virtballoon_free_page_report;
>> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
>> unsigned int capacity;
>> @@ -1044,6 +1043,8 @@ static int virtballoon_probe(struct virtio_device
>> *vdev)
>> goto out_unregister_oom;
>> }
>> + vb->pr_dev_info.report = virtballoon_free_page_report;
>> +
>> /*
>> * The default page reporting order is @pageblock_order, which
>> * corresponds to 512MB in size on ARM64 when 64KB base page
>
>
