Re: [PATCH 01/11] blk-mq: Add blk_mq_init_queue_ops()

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On 3/22/22 13:30, John Garry wrote:
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.

Well; I found that most drivers I had been looking at the scsi command payload isn't used at all; the drivers primarily cared about the (driver-provided) payload, and were completely ignoring the scsi command payload.

Similar for ATA/libsas: you basically never issue real scsi commands, but either 'raw' ATA requests or SCSI TMFs. None of which are scsi commands, so providing them is a bit of a waste.

(And causes irritations, too, as a scsi command requires associated pointers like ->device etc to be set up. Which makes it tricky to use for the initial device setup.)

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.

Yeah, that's what I had in mind.

Cheers,

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

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