Re: [PATCH 3/3] NVMe: Convert to blk-mq

[Keith Busch]

On Fri, 18 Oct 2013, Matias Bjørling wrote:
> On 10/18/2013 05:13 PM, Keith Busch wrote:
> > On Fri, 18 Oct 2013, Matias Bjorling wrote:
> > > The nvme driver implements itself as a bio-based driver. This primarily
> > > because of high lock congestion for high-performance nvm devices. To
> > > remove the congestion within the traditional block layer, a multi-queue
> > > block layer is being implemented.

> > > -    result = nvme_map_bio(nvmeq, iod, bio, dma_dir, psegs);
> > > -    if (result <= 0)
> > > +    if (nvme_map_rq(nvmeq, iod, rq, dma_dir))
> > >         goto free_cmdid;
> > > -    length = result;
> > >
> > > -    cmnd->rw.command_id = cmdid;
> > > +    length = blk_rq_bytes(rq);
> > > +
> > > +    cmnd->rw.command_id = rq->tag;
> >
> > The command ids have to be unique on a submission queue. Since each
> > namespace's blk-mq has its own 'tags' used as command ids here but share
> > submission queues, what's stopping the tags for commands sent to namespace
> > 1 from clashing with tags for namespace 2?
> >
> > I think this would work better if one blk-mq was created per device
> > rather than namespace. It would fix the tag problem above and save a
> > lot of memory potentially wasted on millions of requests allocated that
> > can't be used.
>
> You're right. I didn't see the connection. In v3 I'll push struct 
> request_queue to nvme_dev and map the queues appropriately. It will also fix 
> the command id issues.

Just anticipating a possible issue with the suggestion. Will this separate
the logical block size from the request_queue? Each namespace can have
a different format, so the block size and request_queue can't be tied
together like it currently is for this to work.