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

[Gregory Price]

On Tue, Jun 09, 2026 at 11:53:20AM -0400, Michael S. Tsirkin wrote:
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -174,10 +174,10 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,
>  	 * 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.
> +	 * The division here uses integer division; capacity need
> +	 * not 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);
> 

Initial look - is there a div-by-0 here?  I noticed the old check
prevents this from being (0 * 16), but i don't see (on first pass)
the same check anywhere.

Unless this line below always forces the above to be a
PAGE_REPORTING_CAPCAITY if it's set to 0.

> +	if (!prdev->capacity || prdev->capacity > PAGE_REPORTING_CAPACITY)
> +		prdev->capacity = PAGE_REPORTING_CAPACITY;
> +

It's worth making this corner condition a little more obvious.

The code intends for 

if (capacity == 0)
  capacity = PAGE_REPORTING_CAPACITY

but that's not reflected in the changelog as a default value.

When happens if a driver sets (capacity=0) either on purpose (???) or
because there's a bug (???) and then page_reporting.c forces it up to
32?

There's something to improve here.

~Gregory