Re: [PATCH 05/20] block: Allow bio_iov_iter_get_pages() with bio->bi_bdev unset

[Kent Overstreet]

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.