Re: [PATCH 2/2] blk-mq: ensure a q_usage_counter reference is held when splitting bios

[Christoph Hellwig <hch@lst.de>]

On Thu, Jan 11, 2024 at 10:18:31AM -0700, Jens Axboe wrote:
> This also highlights a potential inefficiency in the patch, as now we're
> grabbing+dropping references when we don't need to. May not be a big
> deal, but it's one of the things that cached requests got rid of. Though
> I'm not quite sure how to refactor to get rid of that, as we'd need to
> shuffle the splitting and request get for that.
> 
> Could you take another look at the series with that in mind?

I thought about it, but it gets pretty ugly quickly.  bio_queue_enter
needs to move back into blk_mq_submit_bio, and then we'd skip it
initially if bio_may_exceed_limits is false, and then we later need
to add it back.  (we'll probably also need to special case
blk_queue_bounce as that setting could change to.  I wish we could
finally kill that)