Re: [PATCH v2] mm: page_reporting: allow driver to set batch capacity

[Gregory Price <gourry@gourry.net>]

On Wed, Jun 10, 2026 at 09:17:08AM -0400, 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 <mst@redhat.com>
> Assisted-by: Claude:claude-opus-4-6

lgtm asside from David's comment request

Reviewed-by: Gregory Price <gourry@gourry.net>

> ---
> 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(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index f6c2dff33f8a..6a1a610c2cb1 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -1017,10 +1017,6 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		unsigned int capacity;
>  
>  		capacity = virtqueue_get_vring_size(vb->reporting_vq);
> -		if (capacity < PAGE_REPORTING_CAPACITY) {
> -			err = -ENOSPC;
> -			goto out_unregister_oom;
> -		}
>  
>  		vb->pr_dev_info.order = PAGE_REPORTING_ORDER_UNSPECIFIED;
>  
> @@ -1041,6 +1037,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		vb->pr_dev_info.order = 5;
>  #endif
>  
> +		vb->pr_dev_info.capacity = capacity;
>  		err = page_reporting_register(&vb->pr_dev_info);
>  		if (err)
>  			goto out_unregister_oom;
> diff --git a/include/linux/page_reporting.h b/include/linux/page_reporting.h
> index 9d4ca5c218a0..048578118a4b 100644
> --- a/include/linux/page_reporting.h
> +++ b/include/linux/page_reporting.h
> @@ -22,6 +22,9 @@ struct page_reporting_dev_info {
>  
>  	/* Minimal order of page reporting */
>  	unsigned int order;
> +
> +	/* Max pages per report batch; 0 (default) means PAGE_REPORTING_CAPACITY */
> +	unsigned int capacity;
>  };
>  
>  /* Tear-down and bring-up for page reporting devices */
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index 7418f2e500bb..942e84b6908a 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -173,11 +173,8 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
>  	 * any pages that may have already been present from the previous
>  	 * list processed. This should result in us reporting all pages on
>  	 * an idle system in about 30 seconds.
> -	 *
> -	 * The division here should be cheap since PAGE_REPORTING_CAPACITY
> -	 * should always be a power of 2.
>  	 */
> -	budget = DIV_ROUND_UP(area->nr_free, PAGE_REPORTING_CAPACITY * 16);
> +	budget = DIV_ROUND_UP(area->nr_free, prdev->capacity * 16);
>  
>  	/* loop through free list adding unreported pages to sg list */
>  	list_for_each_entry_safe(page, next, list, lru) {
> @@ -222,10 +219,10 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
>  		spin_unlock_irq(&zone->lock);
>  
>  		/* begin processing pages in local list */
> -		err = prdev->report(prdev, sgl, PAGE_REPORTING_CAPACITY);
> +		err = prdev->report(prdev, sgl, prdev->capacity);
>  
>  		/* reset offset since the full list was reported */
> -		*offset = PAGE_REPORTING_CAPACITY;
> +		*offset = prdev->capacity;
>  
>  		/* update budget to reflect call to report function */
>  		budget--;
> @@ -234,7 +231,7 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
>  		spin_lock_irq(&zone->lock);
>  
>  		/* flush reported pages from the sg list */
> -		page_reporting_drain(prdev, sgl, PAGE_REPORTING_CAPACITY, !err);
> +		page_reporting_drain(prdev, sgl, prdev->capacity, !err);
>  
>  		/*
>  		 * Reset next to first entry, the old next isn't valid
> @@ -260,13 +257,13 @@ static int
>  page_reporting_process_zone(struct page_reporting_dev_info *prdev,
>  			    struct scatterlist *sgl, struct zone *zone)
>  {
> -	unsigned int order, mt, leftover, offset = PAGE_REPORTING_CAPACITY;
> +	unsigned int order, mt, leftover, offset = prdev->capacity;
>  	unsigned long watermark;
>  	int err = 0;
>  
>  	/* Generate minimum watermark to be able to guarantee progress */
>  	watermark = low_wmark_pages(zone) +
> -		    (PAGE_REPORTING_CAPACITY << page_reporting_order);
> +		    (prdev->capacity << page_reporting_order);
>  
>  	/*
>  	 * Cancel request if insufficient free memory or if we failed
> @@ -290,7 +287,7 @@ page_reporting_process_zone(struct page_reporting_dev_info *prdev,
>  	}
>  
>  	/* report the leftover pages before going idle */
> -	leftover = PAGE_REPORTING_CAPACITY - offset;
> +	leftover = prdev->capacity - offset;
>  	if (leftover) {
>  		sgl = &sgl[offset];
>  		err = prdev->report(prdev, sgl, leftover);
> @@ -322,11 +319,11 @@ static void page_reporting_process(struct work_struct *work)
>  	atomic_set(&prdev->state, state);
>  
>  	/* allocate scatterlist to store pages being reported on */
> -	sgl = kmalloc_objs(*sgl, PAGE_REPORTING_CAPACITY);
> +	sgl = kmalloc_objs(*sgl, prdev->capacity);
>  	if (!sgl)
>  		goto err_out;
>  
> -	sg_init_table(sgl, PAGE_REPORTING_CAPACITY);
> +	sg_init_table(sgl, prdev->capacity);
>  
>  	for_each_zone(zone) {
>  		err = page_reporting_process_zone(prdev, sgl, zone);
> @@ -377,6 +374,9 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>  			page_reporting_order = pageblock_order;
>  	}
>  
> +	if (!prdev->capacity || prdev->capacity > PAGE_REPORTING_CAPACITY)
> +		prdev->capacity = PAGE_REPORTING_CAPACITY;
> +
>  	/* initialize state and work structures */
>  	atomic_set(&prdev->state, PAGE_REPORTING_IDLE);
>  	INIT_DELAYED_WORK(&prdev->work, &page_reporting_process);
> -- 
> MST
>