Re: [PATCHv2] block: enable passthrough command statistics

[Christoph Hellwig <hch@lst.de>]

On Fri, Oct 04, 2024 at 09:04:13AM -0600, Keith Busch wrote:
> Even Jens was a little surprised to find nvme passthrough sets the bio
> bi_bdev. I didn't think it was unusual, but sounds like we are doing
> something special here.

IIRC it was added to support metadata passthrough, but I'd have to do
a little research to find the details.

> > > +	/*
> > > +	 * Ensuring the size is aligned to the block size prevents observing an
> > > +	 * invalid sectors stat.
> > > +	 */
> > > +	if (blk_rq_bytes(req) & (bdev_logical_block_size(bio->bi_bdev) - 1))
> > > +		return false;
> > 
> > Now this probably won't trigger anyway for the usual workload (although
> > it might for odd NVMe command sets like KV and the SLM), but I'd expect the
> > size to be rounded (probably up?) and not entirely dropped.
> 
> This prevents commands with payload sizes that are not representative of
> sector access. Examples from NVMe include Copy, Dataset Management, and
> all the Reservation commands. The transfer size of those commands are
> unlikely to be a block aligned, so it's a simple way to filter them out.
> Rounding the payload size up will produce misleading stats, so I think
> it's better if they don't get to use the feature.

True.  Please put this into the comments!

> 
> > > +	ret = queue_var_store(&ios, page, count);
> > > +	if (ret < 0)
> > > +		return ret;
> > > +
> > > +	if (ios)
> > > +		blk_queue_flag_set(QUEUE_FLAG_IOSTATS_PASSTHROUGH,
> > > +				   disk->queue);
> > > +	else
> > > +		blk_queue_flag_clear(QUEUE_FLAG_IOSTATS_PASSTHROUGH,
> > > +				     disk->queue);
> > 
> > Why is this using queue flags now?  This isn't really blk-mq internal,
> > so it should be using queue_limits->flag as pointed out last round.
> 
> So many flags... The atomic limit update seemed overkill for just this
> flag, but okay.

I've been slowly working on making q->flags entirely limited to
blk-mq internal state.  We're not quite there yet, but I'd like to
keep up the direction rather than having to fix it up later.