Re: [PATCH V2] block: ignore RWF_HIPRI hint for sync dio

[Keith Busch <kbusch@kernel.org>]

On Mon, May 23, 2022 at 09:02:39PM -0600, Keith Busch wrote:
> Here's a bandaid, though I assume it'll break something...

On second thought, maybe this is okay! The encoded hctx doesn't change after
this call, which is the only thing polling cares about. The tag portion doesn't
matter.

The only user for the rq tag part of the cookie is hybrid polling and falls
back to classic polling if the tag wasn't valid, so that scenario is already
hangled. And hybrid polling breaks down anyway if queue-depth is >1, so leaving
an invalid tag might be a good thing.
 
> ---
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ed1869a305c4..348136dc7ba9 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1146,8 +1146,6 @@ void blk_mq_start_request(struct request *rq)
>  	if (blk_integrity_rq(rq) && req_op(rq) == REQ_OP_WRITE)
>  		q->integrity.profile->prepare_fn(rq);
>  #endif
> -	if (rq->bio && rq->bio->bi_opf & REQ_POLLED)
> -	        WRITE_ONCE(rq->bio->bi_cookie, blk_rq_to_qc(rq));
>  }
>  EXPORT_SYMBOL(blk_mq_start_request);
>  
> @@ -2464,6 +2462,9 @@ static void blk_mq_bio_to_request(struct request *rq, struct bio *bio,
>  	WARN_ON_ONCE(err);
>  
>  	blk_account_io_start(rq);
> +
> +	if (rq->bio->bi_opf & REQ_POLLED)
> +	        WRITE_ONCE(rq->bio->bi_cookie, blk_rq_to_qc(rq));
>  }
>  
>  static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
> --