Re: [RFC PATCH V2 09/13] block: use per-task poll context to implement bio based io poll

[Sagi Grimberg <sagi@grimberg.me>]

> +static void blk_bio_poll_post_submit(struct bio *bio, blk_qc_t cookie)
> +{
> +	bio->bi_iter.bi_private_data = cookie;
> +}
> +

Hey Ming, thinking about nvme-mpath, I'm thinking that this should be
an exported function for failover. nvme-mpath updates bio.bi_dev
when re-submitting I/Os to an alternate path, so I'm thinking
that if this function is exported then nvme-mpath could do as little
as the below to allow polling?

--
diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 92adebfaf86f..e562e296153b 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -345,6 +345,7 @@ static void nvme_requeue_work(struct work_struct *work)
         struct nvme_ns_head *head =
                 container_of(work, struct nvme_ns_head, requeue_work);
         struct bio *bio, *next;
+       blk_qc_t cookie;

         spin_lock_irq(&head->requeue_lock);
         next = bio_list_get(&head->requeue_list);
@@ -359,7 +360,8 @@ static void nvme_requeue_work(struct work_struct *work)
                  * path.
                  */
                 bio_set_dev(bio, head->disk->part0);
-               submit_bio_noacct(bio);
+               cookie = submit_bio_noacct(bio);
+               blk_bio_poll_post_submit(bio, cookie);
         }
  }
--

I/O failover will create misalignment from the polling context cpu and
the submission cpu (running requeue_work), but I don't see if there is
something that would break here...

Thoughts?