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
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SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
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