Re: [PATCHv3] blk-mq: pop cached request if it is usable

[Keith Busch <kbusch@kernel.org>]

On Fri, May 22, 2026 at 11:03:40AM +0200, Christoph Hellwig wrote:
> > @@ -3257,12 +3243,10 @@ void blk_mq_submit_bio(struct bio *bio)
> >  	return;
> >  
> >  queue_exit:
> > -	/*
> > -	 * Don't drop the queue reference if we were trying to use a cached
> > -	 * request and thus didn't acquire one.
> > -	 */
> >  	if (!rq)
> >  		blk_queue_exit(q);
> > +	else
> > +		blk_mq_free_request(rq);
> >  }
> 
> I think keeping a comment here would be nice, a would be avoid
> the inversion of the condition for a trivial if/else.
> 
> But on a higher level, I really think you should add it back to the
> batch list here, otherwise we're doing lots of roundtrips through
> blk_mq_free_request for trivially mergable sequential I/O.

That gets compicated when we did sleep. This request may be the last
reference that's holding up a freeze. We really want to re-enter the
queue in that case because the driver is trying to change the queue
limits. Unless this is measurably harming perforamnce, it's a corner
case that's just easier to not think about.