Re: [PATCH 4/4] nvme: add support for mq_ops->queue_rqs()

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 12/16/2021 5:59 PM, Jens Axboe wrote:
> On 12/16/21 6:02 AM, Max Gurtovoy wrote:
>> On 12/15/2021 6:24 PM, Jens Axboe wrote:
>>> This enables the block layer to send us a full plug list of requests
>>> that need submitting. The block layer guarantees that they all belong
>>> to the same queue, but we do have to check the hardware queue mapping
>>> for each request.
>>>
>>> If errors are encountered, leave them in the passed in list. Then the
>>> block layer will handle them individually.
>>>
>>> This is good for about a 4% improvement in peak performance, taking us
>>> from 9.6M to 10M IOPS/core.
>>>
>>> Reviewed-by: Hannes Reinecke <hare@suse.de>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> ---
>>>    drivers/nvme/host/pci.c | 61 +++++++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 61 insertions(+)
>>>
>>> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
>>> index 6be6b1ab4285..197aa45ef7ef 100644
>>> --- a/drivers/nvme/host/pci.c
>>> +++ b/drivers/nvme/host/pci.c
>>> @@ -981,6 +981,66 @@ static blk_status_t nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
>>>    	return BLK_STS_OK;
>>>    }
>>>    
>>> +static void nvme_submit_cmds(struct nvme_queue *nvmeq, struct request **rqlist)
>>> +{
>>> +	spin_lock(&nvmeq->sq_lock);
>>> +	while (!rq_list_empty(*rqlist)) {
>>> +		struct request *req = rq_list_pop(rqlist);
>>> +		struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
>>> +
>>> +		memcpy(nvmeq->sq_cmds + (nvmeq->sq_tail << nvmeq->sqes),
>>> +				absolute_pointer(&iod->cmd), sizeof(iod->cmd));
>>> +		if (++nvmeq->sq_tail == nvmeq->q_depth)
>>> +			nvmeq->sq_tail = 0;
>>> +	}
>>> +	nvme_write_sq_db(nvmeq, true);
>>> +	spin_unlock(&nvmeq->sq_lock);
>>> +}
>>> +
>>> +static bool nvme_prep_rq_batch(struct nvme_queue *nvmeq, struct request *req)
>>> +{
>>> +	/*
>>> +	 * We should not need to do this, but we're still using this to
>>> +	 * ensure we can drain requests on a dying queue.
>>> +	 */
>>> +	if (unlikely(!test_bit(NVMEQ_ENABLED, &nvmeq->flags)))
>>> +		return false;
>>> +	if (unlikely(!nvme_check_ready(&nvmeq->dev->ctrl, req, true)))
>>> +		return false;
>>> +
>>> +	req->mq_hctx->tags->rqs[req->tag] = req;
>>> +	return nvme_prep_rq(nvmeq->dev, req) == BLK_STS_OK;
>>> +}
>>> +
>>> +static void nvme_queue_rqs(struct request **rqlist)
>>> +{
>>> +	struct request *req = rq_list_peek(rqlist), *prev = NULL;
>>> +	struct request *requeue_list = NULL;
>>> +
>>> +	do {
>>> +		struct nvme_queue *nvmeq = req->mq_hctx->driver_data;
>>> +
>>> +		if (!nvme_prep_rq_batch(nvmeq, req)) {
>>> +			/* detach 'req' and add to remainder list */
>>> +			if (prev)
>>> +				prev->rq_next = req->rq_next;
>>> +			rq_list_add(&requeue_list, req);
>>> +		} else {
>>> +			prev = req;
>>> +		}
>>> +
>>> +		req = rq_list_next(req);
>>> +		if (!req || (prev && req->mq_hctx != prev->mq_hctx)) {
>>> +			/* detach rest of list, and submit */
>>> +			prev->rq_next = NULL;
>> if req == NULL and prev == NULL we'll get a NULL deref here.
>>
>> I think this can happen in the first iteration.
>>
>> Correct me if I'm wrong..
> First iteration we know the list isn't empty, so req can't be NULL
> there.

but you set "req = rq_list_next(req);"

So can't req be NULL ? after the above line ?

>