Re: [PATCH v2 for-6.18/block 05/10] blk-mq: cleanup shared tags case in blk_mq_update_nr_requests()

[Chris Mason <clm@meta.com>]

Hi everyone,

My review automation flagged this one in linux-next, and it looks like a
valid bug to me:

On Wed, 10 Sep 2025 16:04:40 +0800 Yu Kuai <yukuai1@huaweicloud.com> wrote:

> From: Yu Kuai <yukuai3@huawei.com>
> 
> For shared tags case, all hctx->sched_tags/tags are the same, it doesn't
> make sense to call into blk_mq_tag_update_depth() multiple times for the
> same tags.
> 

[ ... ]

> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 9b97f2f3f2c9..80c20700bce8 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -4935,34 +4935,35 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
>
>  	blk_mq_quiesce_queue(q);
>
> -	queue_for_each_hw_ctx(q, hctx, i) {
> -		if (!hctx->tags)
> -			continue;
> -		/*
> -		 * If we're using an MQ scheduler, just update the scheduler
> -		 * queue depth. This is similar to what the old code would do.
> -		 */
> -		if (hctx->sched_tags) {
> -			ret = blk_mq_tag_update_depth(hctx, &hctx->sched_tags,
> -						      nr);
> -		} else {
> -			ret = blk_mq_tag_update_depth(hctx, &hctx->tags, nr);
> -		}
> -		if (ret)
> -			goto out;
> -	}
> -
> -	q->nr_requests = nr;
> -	if (q->elevator && q->elevator->type->ops.depth_updated)
> -		q->elevator->type->ops.depth_updated(q);
> -
>  	if (blk_mq_is_shared_tags(set->flags)) {
>  		if (q->elevator)
>  			blk_mq_tag_update_sched_shared_tags(q);
                        ^^^^

Does blk_mq_tag_update_sched_shared_tags() use the wrong value here?

In the old code, q->nr_requests was updated to nr before calling
blk_mq_tag_update_sched_shared_tags(). But in the new code, this
function is called while q->nr_requests still contains the old value.

Looking at blk_mq_tag_update_sched_shared_tags():

    void blk_mq_tag_update_sched_shared_tags(struct request_queue *q)
    {
        sbitmap_queue_resize(&q->sched_shared_tags->bitmap_tags,
                             q->nr_requests - q->tag_set->reserved_tags);
    }

It reads q->nr_requests to calculate the new sbitmap size. With the
reordering, this will resize the sbitmap to the old depth instead of
the new depth passed in nr.

-chris