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

[John Garry]

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