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Date:   Tue, 22 Mar 2022 12:30:42 +0000
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Subject: Re: [PATCH 01/11] blk-mq: Add blk_mq_init_queue_ops()
To:     Hannes Reinecke <hare@suse.de>, Christoph Hellwig <hch@lst.de>
CC:     <axboe@kernel.dk>, <damien.lemoal@opensource.wdc.com>,
        <bvanassche@acm.org>, <jejb@linux.ibm.com>,
        <martin.petersen@oracle.com>, <ming.lei@redhat.com>,
        <chenxiang66@hisilicon.com>, <linux-block@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-ide@vger.kernel.org>,
        <linux-scsi@vger.kernel.org>, <dm-devel@redhat.com>,
        <beanhuo@micron.com>
References: <1647945585-197349-1-git-send-email-john.garry@huawei.com>
 <1647945585-197349-2-git-send-email-john.garry@huawei.com>
 <20220322111848.GA29270@lst.de>
 <cacc3f7b-c8be-0f72-1c52-562c15b468a4@huawei.com>
 <b5df2ef1-2d6d-340e-e4b4-09132dc0516b@suse.de>
From:   John Garry <john.garry@huawei.com>
In-Reply-To: <b5df2ef1-2d6d-340e-e4b4-09132dc0516b@suse.de>
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On 22/03/2022 12:16, Hannes Reinecke wrote:
> On 3/22/22 12:33, John Garry wrote:
>> On 22/03/2022 11:18, Christoph Hellwig wrote:
>>> On Tue, Mar 22, 2022 at 06:39:35PM +0800, John Garry wrote:
>>>> Add an API to allocate a request queue which accepts a custom set of
>>>> blk_mq_ops for that request queue.
>>>>
>>>> The reason which we may want custom ops is for queuing requests 
>>>> which we
>>>> don't want to go through the normal queuing path.
>>>
>>> Eww.  I really do not think we should do separate ops per queue, as that
>>> is going to get us into a deep mess eventually.
>>>
>>
>> Yeah... so far (here) it works out quite nicely, as we don't need to 
>> change the SCSI blk mq ops nor allocate a scsi_device - everything is 
>> just separate.
>>
>> The other method mentioned previously was to add the request 
>> "reserved" flag and add new paths in scsi_queue_rq() et al to handle 
>> this, but that gets messy.
>>
>> Any other ideas ...?
>>
> 
> As outlined in the other mail, I think might be useful is to have a 
> _third_ type of requests (in addition to the normal and the reserved ones).
> That one would be allocated from the normal I/O pool (and hence could 
> fail if the pool is exhausted), but would be able to carry a different 
> payload (type) than the normal requests.

As mentioned in the cover letter response, it just seems best to keep 
the normal scsi_cmnd payload but have other means to add on the internal 
command data, like using host_scribble or scsi_cmnd priv data.

> And we could have a separate queue_rq for these requests, as we can 
> differentiate them in the block layer.

I don't know, let me think about it. Maybe we could add an "internal" 
blk flag, which uses a separate "internal" queue_rq callback.

Thanks,
John