Re: [PATCH 3/4] block: move queue enter logic into blk_mq_submit_bio()

[Christoph Hellwig <hch@infradead.org>]

On Thu, Nov 04, 2021 at 05:41:35AM -0600, Jens Axboe wrote:
> >>  	if (!submit_bio_checks(bio) || !blk_crypto_bio_prep(&bio))
> >> -		goto queue_exit;
> >> +		return;
> > 
> > This is broken, we really ant the submit checks under freeze
> > protection to make sure the parameters can't be changed underneath
> > us.
> 
> Which parameters are you worried about in submit_bio_checks()? I don't
> immediately see anything that would make me worry about it.

Mostly checks if certain operations are supported or not, as
revalidation could clear those.

> > This looks weird, as blk_try_enter_queue is already called by
> > bio_queue_enter.
> 
> It's just for avoiding a pointless call into bio_queue_enter(), which
> isn't needed it blk_try_enter_queue() is successful. The latter is short
> and small and can be inlined, while bio_queue_enter() is a lot bigger.

If this is so impotant let's operated with an inlined bio_queue_enter
that calls out of line into slow path instead of open coding it
like this.

> >>  	} else {
> >>  		struct blk_mq_alloc_data data = {
> >>  			.q		= q,
> >> @@ -2528,6 +2534,11 @@ void blk_mq_submit_bio(struct bio *bio)
> >>  			.cmd_flags	= bio->bi_opf,
> >>  		};
> >>  
> >> +		if (unlikely(!blk_mq_queue_enter(q, bio)))
> >> +			return;
> >> +
> >> +		rq_qos_throttle(q, bio);
> >> +
> > 
> > At some point the code in this !cached branch really needs to move
> > into a helper..
> 
> Like in the next patch?

No, I mean the !cached case which is a lot more convoluted.