On Wed, Jan 29, 2020 at 09:01:49PM -0700, Jens Axboe wrote:

blk_poll() used to be a pointer in the queue, but since we just had
one implementation, we got rid of it. Might make sense to
reintroduce that, and just make it an optimized indirect call. I
think that would be prettier than add the bio hack in the middle of
it, and you're having to add a queue pointer anyway.

Well, the other reason is to avoid an indirect call for the blk-mq
case, which are fairly expensive.  In fact I'm pretty sure we avoid
indirect calls from the bio layer into blk-mq entirely for the fast
path at the moment, and it would be great to keep it that way.