linux-kernel - Re: [PATCH 05/20] block: Allow bio_iov_iter_get_pages() with bio->bi

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Date:   Wed, 2 Aug 2023 04:47:18 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-bcachefs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 05/20] block: Allow bio_iov_iter_get_pages() with
 bio->bi_bdev unset

On Tue, Aug 01, 2023 at 03:04:50PM -0400, Kent Overstreet wrote:
> > 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?

Hey, ask the cgroup folks as they come up with it.  I'm not going to
defend the logic here.

> 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().

It's checking bi_blkg because it can also be called from bio_set_dev.

> And this is your code, so I think you need to go over this again.

It's "my code" in the sene of that I did one big round of unwinding
the even bigger mess that was there.  There is another few rounds needed
for the code to vaguely make sense.