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 10:04 AM, Omar Sandoval wrote:
> On Mon, Feb 27, 2017 at 10:03:29AM -0700, Jens Axboe wrote:
>> 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.
> 
> Will do, I'll make sure to add Sagi's reported-by.

Thanks. Sagi, I updated your first patch as follows:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=d06f713e5d200959cdb445a0104e71d9e6070c51

and this is now head of for-linus.

-- 
Jens Axboe