linux-kernel - Re: [PATCH v2 for-6.18/block 05/10] blk-mq: cleanup shared tags case in blk_mq_update_nr

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Message-ID: <fd66fda2-8dd9-4009-9c4b-7cebaac64c05@linux.ibm.com>
Date: Tue, 14 Oct 2025 23:34:02 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Chris Mason <clm@...a.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc: axboe@...nel.dk, ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com, yi.zhang@...wei.com,
        yangerkun@...wei.com, johnny.chenyi@...wei.com
Subject: Re: [PATCH v2 for-6.18/block 05/10] blk-mq: cleanup shared tags case
 in blk_mq_update_nr_requests()



On 10/14/25 6:35 PM, Chris Mason wrote:
> 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@...weicloud.com> wrote:
> 
>> From: Yu Kuai <yukuai3@...wei.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.
> 
Good catch! Yes, I think this needs to be fixed...

Thanks,
--Nilay