linux-kernel - Re: [PATCH 3/3] NVMe: Convert to blk-mq

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Date:	Tue, 22 Oct 2013 20:55:37 +0200
From:	Matias Bjorling <m@...rling.me>
To:	Keith Busch <keith.busch@...el.com>, axboe@...nel.dk
CC:	willy@...ux.intel.com, linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.org
Subject: Re: [PATCH 3/3] NVMe: Convert to blk-mq

Den 22-10-2013 18:55, Keith Busch skrev:
> On Fri, 18 Oct 2013, Matias Bjørling wrote:
>> On 10/18/2013 05:13 PM, Keith Busch wrote:
>>> On Fri, 18 Oct 2013, Matias Bjorling wrote:
>>>> The nvme driver implements itself as a bio-based driver. This 
>>>> primarily
>>>> because of high lock congestion for high-performance nvm devices. To
>>>> remove the congestion within the traditional block layer, a 
>>>> multi-queue
>>>> block layer is being implemented.
>
>>>> -    result = nvme_map_bio(nvmeq, iod, bio, dma_dir, psegs);
>>>> -    if (result <= 0)
>>>> +    if (nvme_map_rq(nvmeq, iod, rq, dma_dir))
>>>>         goto free_cmdid;
>>>> -    length = result;
>>>>
>>>> -    cmnd->rw.command_id = cmdid;
>>>> +    length = blk_rq_bytes(rq);
>>>> +
>>>> +    cmnd->rw.command_id = rq->tag;
>>>
>>> The command ids have to be unique on a submission queue. Since each
>>> namespace's blk-mq has its own 'tags' used as command ids here but 
>>> share
>>> submission queues, what's stopping the tags for commands sent to 
>>> namespace
>>> 1 from clashing with tags for namespace 2?
>>>
>>> I think this would work better if one blk-mq was created per device
>>> rather than namespace. It would fix the tag problem above and save a
>>> lot of memory potentially wasted on millions of requests allocated that
>>> can't be used.
>>
>> You're right. I didn't see the connection. In v3 I'll push struct 
>> request_queue to nvme_dev and map the queues appropriately. It will 
>> also fix the command id issues.
>
> Just anticipating a possible issue with the suggestion. Will this 
> separate
> the logical block size from the request_queue? Each namespace can have
> a different format, so the block size and request_queue can't be tied
> together like it currently is for this to work.

If only a couple of different logical sizes are to be expected (1-4), we 
can keep a list of already initialized request queues, and use the one 
that match an already initialized?

Axboe, do you know of a better solution?
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