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

[Hannes Reinecke]

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
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