On Wed, May 13, 2020 at 02:21:47PM +0200, Christoph Hellwig wrote:
> Use of the BLK_MQ_REQ_FORCE is pretty bogus here..
>
> > + if (rq->rq_flags & RQF_PREEMPT)
> > + flags |= BLK_MQ_REQ_PREEMPT;
> > + if (reserved)
> > + flags |= BLK_MQ_REQ_RESERVED;
> > + /*
> > + * Queue freezing might be in-progress, and wait freeze can't be
> > + * done now because we have request not completed yet, so mark this
> > + * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
> > + * freeze hung forever.
> > + */
> > + flags |= BLK_MQ_REQ_FORCE;
> > +
> > + /* avoid allocation failure by clearing NOWAIT */
> > + nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
> > + if (!nrq)
> > + return;
>
> blk_get_request returns an ERR_PTR.
>
> But I'd rather avoid the magic new BLK_MQ_REQ_FORCE hack when we can
> just open code it and document what is going on:
BLK_MQ_REQ_FORCE is actually not a hack, there are other use cases
which need that too, see commit log of patch 10/12.
>
> static struct blk_mq_tags *blk_mq_rq_tags(struct request *rq)
> {
> struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
>
> if (rq->q->elevator)
> return hctx->sched_tags;
> return hctx->tags;
> }
>
> static void blk_mq_resubmit_rq(struct request *rq)
> {
> struct blk_mq_alloc_data alloc_data = {
> .cmd_flags = rq->cmd_flags & ~REQ_NOWAIT;
> };
> struct request *nrq;
>
> if (rq->rq_flags & RQF_PREEMPT)
> alloc_data.flags |= BLK_MQ_REQ_PREEMPT;
> if (blk_mq_tag_is_reserved(blk_mq_rq_tags(rq), rq->internal_tag))
> alloc_data.flags |= BLK_MQ_REQ_RESERVED;
>
> /*
> * We must still be able to finish a resubmission due to a hotplug
> * even even if a queue freeze is in progress.
> */
> percpu_ref_get(&q->q_usage_counter);
> nrq = blk_mq_get_request(rq->q, NULL, &alloc_data);
> blk_queue_exit(q);
This way works too.
>
> if (!nrq)
> return; // XXX: warn?
It isn't possible because we clears NO_WAIT flag.
Thanks,
Ming
