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Date:   Tue, 22 Mar 2022 13:16:46 +0100
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To:     John Garry <john.garry@huawei.com>, Christoph Hellwig <hch@lst.de>
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        bvanassche@acm.org, jejb@linux.ibm.com, martin.petersen@oracle.com,
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        linux-ide@vger.kernel.org, linux-scsi@vger.kernel.org,
        dm-devel@redhat.com, beanhuo@micron.com
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From:   Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH 01/11] blk-mq: Add blk_mq_init_queue_ops()
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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.
And we could have a separate queue_rq for these requests, as we can 
differentiate them in the block layer.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), GF: Felix Imendörffer