Re: [PATCH 2/2] blk-mq: make sure to back-assign the request to rq_map in blk_mq_alloc_request_hctx

[Jens Axboe <axboe@kernel.dk>]

On 02/27/2017 09:59 AM, Omar Sandoval wrote:
> On Mon, Feb 27, 2017 at 05:36:21PM +0200, Sagi Grimberg wrote:
>> Otherwise we won't be able to retrieve the request from
>> the tag.
>>
>> Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
>> ---
>>  block/blk-mq.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index d84c66fb37b7..9611cd9920e9 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -312,6 +312,7 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q, int rw,
>>  		ret = -EWOULDBLOCK;
>>  		goto out_queue_exit;
>>  	}
>> +	alloc_data.hctx->tags->rqs[rq->tag] = rq;
>>  
>>  	return rq;
>>  
>> -- 
>> 2.7.4
> 
> This one I think is a little bit cleaner if we just push that assignment
> into __blk_mq_alloc_request() like this (again, compile tested only):
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 98c7b061781e..7267c9c23529 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -135,8 +135,6 @@ struct request *blk_mq_sched_get_request(struct request_queue *q,
>  			rq = __blk_mq_alloc_request(data, op);
>  	} else {
>  		rq = __blk_mq_alloc_request(data, op);
> -		if (rq)
> -			data->hctx->tags->rqs[rq->tag] = rq;
>  	}
>  
>  	if (rq) {
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 9e6b064e5339..b4cf9dfa926b 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -234,6 +234,7 @@ struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data,
>  			}
>  			rq->tag = tag;
>  			rq->internal_tag = -1;
> +			data->hctx->tags->rqs[rq->tag] = rq;
>  		}
>  
>  		blk_mq_rq_ctx_init(data->q, data->ctx, rq, op);

Agree, let's keep that in one place, if we can.

> Looking a little closer at the caller, though, this is kind of weird:
> 
> struct request *nvme_alloc_request(struct request_queue *q,
> 		struct nvme_command *cmd, unsigned int flags, int qid)
> {
> 	unsigned op = nvme_is_write(cmd) ? REQ_OP_DRV_OUT : REQ_OP_DRV_IN;
> 	struct request *req;
> 
> 	if (qid == NVME_QID_ANY) {
> 		req = blk_mq_alloc_request(q, op, flags);
> 	} else {
> 		req = blk_mq_alloc_request_hctx(q, op, flags,
> 				qid ? qid - 1 : 0);
> 	}
> 	if (IS_ERR(req))
> 		return req;
> 
> 	req->cmd_flags |= REQ_FAILFAST_DRIVER;
> 	nvme_req(req)->cmd = cmd;
> 
> 	return req;
> }
> 
> In the "any" case, we allocate a request with a scheduler tag and go
> through the scheduler as usual. In the hctx case, we're getting a
> request with a driver tag, meaning we go through the
> blk_mq_sched_bypass_insert() path when we run the request.
> 
> There's nothing really wrong about that, it just seems weird. Not sure
> if it's weird enough to act on :)

That's just broken, we need to fix that up. _hctx() request alloc
should return scheduler request as well.

Omar, care to rework patch #1 and incorporate a fix for the hctx
alloc? Then I'll fix up patch #2, adding the carry-over of the
reserved flag. We'll just rebase for-linus, it's not a stable
branch.

-- 
Jens Axboe