Re: [PATCH v14 5/8] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Aaron Tomlin]

On Wed, May 20, 2026 at 05:50:27PM -0400, Aaron Tomlin wrote:
> diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> index 705da074ad6c..3df5c368cc08 100644
> --- a/block/blk-mq-cpumap.c
> +++ b/block/blk-mq-cpumap.c
> @@ -22,7 +22,11 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
>  {
>  	unsigned int num;
>  
> -	num = cpumask_weight(mask);
> +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
> +		num = cpumask_weight_and(mask, housekeeping_cpumask(HK_TYPE_IO_QUEUE));
> +	else
> +		num = cpumask_weight(mask);
> +
>  	return min_not_zero(num, max_queues);
>  }

I think we could be more defensive here.

It is theoretically possible for the intersection of the provided mask and
the housekeeping mask to hit 0. We cannot allow a generic block layer
utility function to overflow to maximum queues. Therefore, if the
intersection evaluates to 0, we should enforce a safe fallback of 1:

@@ -26,7 +26,11 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
 	else
 		num = cpumask_weight(mask);
 
-	return min_not_zero(num, max_queues);
+	/*
+	 * Ensure that a count of zero does not inadvertently result in
+	 * allocating the maximum number of queues.
+	 */
+	return min_not_zero(num ?: 1U, max_queues);
 }


-- 
Aaron Tomlin