On Wed, Jul 26, 2023 at 06:23:05AM -0700, Christoph Hellwig wrote: > On Mon, Jul 24, 2023 at 10:43:12PM -0400, Kent Overstreet wrote: > > Doing the blk-cgroup association at bio alloc time sounds broken to me, > > because of stacking block devices - why was the association not done at > > generic_make_request() time? > > Because blk-cgroup not only works at the lowest level in the stack, > but also for stackable block devices. It's not a design decision I > particularly agree with, but it's been there forever. You're setting the association only to the highest block device in the stack - how on earth is it supposed to work with anything lower? And looking at bio_associate_blkg(), this code looks completely broken. It's checking bio->bi_blkg, but that's just been set to NULL in bio_init(). And this is your code, so I think you need to go over this again. Anyways, bio_associate_blkg() is also called by bio_set_dev(), which means passing the block device to bio_init() was a completely pointless change. bcachefs uses bio_set_dev(), so everything is fine.
