[PATCH 2/2] nvme-multipath: don't block on blk_queue_enter of the underlying device

[Sagi Grimberg]

> Actually, I had been playing around with marking the entire bio as 
> 'NOWAIT'; that would avoid the tag stall, too:
> 
> @@ -313,7 +316,7 @@ blk_qc_t nvme_ns_head_submit_bio(struct bio *bio)
>          ns = nvme_find_path(head);
>          if (likely(ns)) {
>                  bio_set_dev(bio, ns->disk->part0);
> -               bio->bi_opf |= REQ_NVME_MPATH;
> +               bio->bi_opf |= REQ_NVME_MPATH | REQ_NOWAIT;
>                  trace_block_bio_remap(bio, disk_devt(ns->head->disk),
>                                        bio->bi_iter.bi_sector);
>                  ret = submit_bio_noacct(bio);
> 
> 
> My only worry here is that we might incur spurious failures under high 
> load; but then this is not necessarily a bad thing.

What? making spurious failures is not ok under any load. what fs will
take into account that you may have run out of tags?