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Date:   Tue, 1 Aug 2023 15:04:50 -0400
From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     linux-bcachefs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org
Subject: Re: [PATCH 05/20] block: Allow bio_iov_iter_get_pages() with
 bio->bi_bdev unset
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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.