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

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Message-ID: <CACVXFVOxx973ho9yN0W95YCj2XQBoYqyu4aD4bZ_9Pv89XGgnQ@mail.gmail.com>
Date:	Thu, 5 Jun 2014 00:26:15 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
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 Thu, Jun 5, 2014 at 12:09 AM, Jens Axboe <axboe@...nel.dk> wrote:
> 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.

Attachment patch should be enough.

Thanks,
-- 
Ming Lei

View attachment "clear_flush_rq_tag.patch" of type "text/x-patch" (394 bytes)