Re: [PATCH] block: fix direct dispatch issue failure for clones

["jianchao.wang" <jianchao.w.wang@oracle.com>]

On 12/7/18 6:20 AM, Jens Axboe wrote:
> After the direct dispatch corruption fix, we permanently disallow direct
> dispatch of non read/write requests. This works fine off the normal IO
> path, as they will be retried like any other failed direct dispatch
> request. But for the blk_insert_cloned_request() that only DM uses to
> bypass the bottom level scheduler, we always first attempt direct
> dispatch. For some types of requests, that's now a permanent failure,
> and no amount of retrying will make that succeed.
> 
> Don't use direct dispatch off the cloned insert path, always just use
> bypass inserts. This still bypasses the bottom level scheduler, which is
> what DM wants.
> 
> Fixes: ffe81d45322c ("blk-mq: fix corruption with direct issue")
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index deb56932f8c4..4c44e6fa0d08 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2637,7 +2637,8 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
>  		 * bypass a potential scheduler on the bottom device for
>  		 * insert.
>  		 */
> -		return blk_mq_request_issue_directly(rq);
> +		blk_mq_request_bypass_insert(rq, true);
> +		return BLK_STS_OK;
>  	}
>  
>  	spin_lock_irqsave(q->queue_lock, flags);
> 
Not sure about this because it will break the merging promotion for request based DM
from Ming.
396eaf21ee17c476e8f66249fb1f4a39003d0ab4
(blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback)

We could use some other way to fix this.

Thanks
Jianchao