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

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Date:   Mon, 24 Jul 2023 22:43:12 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     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 Mon, Jul 24, 2023 at 10:34:20AM -0700, Christoph Hellwig wrote:
> On Wed, Jul 12, 2023 at 05:11:00PM -0400, Kent Overstreet wrote:
> > bio_iov_iter_get_pages() trims the IO based on the block size of the
> > block device the IO will be issued to.
> > 
> > However, bcachefs is a multi device filesystem; when we're creating the
> > bio we don't yet know which block device the bio will be submitted to -
> > we have to handle the alignment checks elsewhere.
> 
> So, we've been trying really hard to always make sure to pass a bdev
> to anything that allocates a bio, mostly due due the fact that we
> actually derive information like the blk-cgroup associations from it.
> 
> The whole blk-cgroup stuff is actually a problem for non-trivial
> multi-device setups.  XFS gets away fine because each file just
> sits on either the main or RT device and no user I/O goes to the
> log device, and btrfs papers over it in a weird way by always
> associating with the last added device, which is in many ways gross
> and wrong, but at least satisfies the assumptions made in blk-cgroup.
> 
> How do you plan to deal with this?  Because I really don't want folks
> just to go ahead and ignore the issues, we need to actually sort this
> out.

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?