Re: [PATCH] block: Fix blk_mq_try_issue_directly()

[Ming Lei <ming.lei@redhat.com>]

On Wed, Apr 03, 2019 at 01:11:26PM -0700, Bart Van Assche wrote:
> If blk_mq_try_issue_directly() returns BLK_STS*_RESOURCE that means that
> the request has not been queued and that the caller should retry to submit
> the request. Both blk_mq_request_bypass_insert() and
> blk_mq_sched_insert_request() guarantee that a request will be processed.
> Hence return BLK_STS_OK if one of these functions is called. This patch
> avoids that blk_mq_dispatch_rq_list() crashes when using dm-mpath.
> 
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: James Smart <james.smart@broadcom.com>
> Cc: Ming Lei <ming.lei@redhat.com>
> Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
> Cc: Keith Busch <keith.busch@intel.com>
> Cc: Dongli Zhang <dongli.zhang@oracle.com>
> Cc: Laurence Oberman <loberman@redhat.com>
> Tested-by: Laurence Oberman <loberman@redhat.com>
> Reviewed-by: Laurence Oberman <loberman@redhat.com>
> Reported-by: Laurence Oberman <loberman@redhat.com>
> Fixes: 7f556a44e61d ("blk-mq: refactor the code of issue request directly") # v5.0.
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> ---
>  block/blk-mq.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 652d0c6d5945..b2c20dce8a30 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1859,16 +1859,11 @@ blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	case BLK_STS_RESOURCE:
>  		if (force) {
>  			blk_mq_request_bypass_insert(rq, run_queue);
> -			/*
> -			 * We have to return BLK_STS_OK for the DM
> -			 * to avoid livelock. Otherwise, we return
> -			 * the real result to indicate whether the
> -			 * request is direct-issued successfully.
> -			 */
> -			ret = bypass ? BLK_STS_OK : ret;
> +			ret = BLK_STS_OK;
>  		} else if (!bypass) {
>  			blk_mq_sched_insert_request(rq, false,
>  						    run_queue, false);
> +			ret = BLK_STS_OK;
>  		}

This change itself is correct.

However, there is other issue introduced by 7f556a44e61d.

We need blk_insert_cloned_request() to pass back BLK_STS_RESOURCE/BLK_STS_RESOURCE
to caller, so that dm-rq driver may see the underlying queue is busy, then tell
blk-mq to deal with the busy condition from dm-rq queue, so that IO
merge can get improved.

That is exactly what 396eaf21ee17c476e8f6 ("blk-mq: improve DM's blk-mq IO merging
via blk_insert_cloned_request feedback") did.

There must be performance regression with 7f556a44e61d by cut the feedback.

So could you fix them all in one patch?


Thanks,
Ming