linux-kernel - Re: [patch]blk-mq: blk_mq_tag_to

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Message-ID: <538F44AD.4090906@kernel.dk>
Date:	Wed, 04 Jun 2014 10:09:17 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <tom.leiming@...il.com>
CC:	Christoph Hellwig <hch@...radead.org>,
	Shaohua Li <shli@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch]blk-mq: blk_mq_tag_to_rq should handle flush request

On 06/04/2014 10:00 AM, Ming Lei wrote:
> On Wed, Jun 4, 2014 at 11:48 PM, Jens Axboe <axboe@...nel.dk> wrote:
>> On 06/04/2014 09:43 AM, Ming Lei wrote:
>>> On Wed, Jun 4, 2014 at 11:31 PM, Christoph Hellwig <hch@...radead.org> wrote:
>>>> On Wed, Jun 04, 2014 at 09:02:19AM -0600, Jens Axboe wrote:
>>>>>> scsi_mq_find_tag only gets the scsi host, which may have multiple
>>>>>> queues.  When called from scsi_find_tag we actually have a scsi device,
>>>>>> so that's not an issue, but when called from scsi_host_find_tag the
>>>>>> driver only provides the host.
>>>>>
>>>>> Only solution I see right now is to have the flush_rq in the shared
>>>>> tags, but that would potentially be a regression for multiple
>>>>> devices and heavy flush uses cases. I'll see if I can come up with
>>>>> something better, or maybe Shaohua has an idea.
>>>>
>>>> What about something like the following (untest, uncompiled, maybe
>>>> pseudo-code):
>>>>
>>>> struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
>>>> {
>>>>         struct request *rq = tags->rqs[tag];
>>>>
>>>>         if ((rq->cmd_flags & REQ_FLUSH_SEQ) && rq->q->flush_rq->tag == tag)
>>>>                 return rq->q->flush_rq;
>>>>         return rq;
>>>> }
>>>
>>> Looks we thought it together, :-)
>>>
>>> Also maybe the flush_rq->tag need to be cleared in flush_end_io().
>>
>> It clears the command flag, so that should be enough.
> 
> Only the flush_rq's command flag is cleared, and its parent request
> flag isn't cleared.

Good point. Care to send in a patch? We can just clear it to -1U, at
least in blk-mq that's defined as an invalid tag.

-- 
Jens Axboe

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