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

[Christoph Hellwig]

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.

So we need to assign it when creating the bio (we used to do it at
submission time, but the way it was done was horribly ineffcient,
that's why I moved it to allocation time), and then when hitting a
stacked device it get reassinged (which still is horribly inefficient).