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

[Yu Kuai]

Hi,

在 2025/10/15 2:04, Nilay Shroff 写道:
> 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@xxxxxxxxxxxxxxx> wrote:
> >
> > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > >
> > > 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...

Yeah, I'll send a fix ASAP.

Thanks,
Kuai

> Thanks,
> --Nilay
>
>
> .