Re: [PATCH 1/1] virtio_balloon: do not set pr_dev_info.report unconditionally

[Dongli Zhang <dongli.zhang@oracle.com>]

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 <dongli.zhang@oracle.com>
>> ---
>>   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
> 
>