[dm-devel] [PATCH 5/8] dm: always setup ->orig_bio in alloc_io

[Mike Snitzer]

On Tue, Apr 12 2022 at  4:56P -0400,
Ming Lei <ming.lei at redhat.com> wrote:

> The current DM codes setup ->orig_bio after __map_bio() returns,
> and not only cause kernel panic for dm zone, but also a bit ugly
> and tricky, especially the waiting until ->orig_bio is set in
> dm_submit_bio_remap().
> 
> The reason is that one new bio is cloned from original FS bio to
> represent the mapped part, which just serves io accounting.
> 
> Now we have switched to bdev based io accounting interface, and we
> can retrieve sectors/bio_op from both the real original bio and the
> added fields of .sector_offset & .sectors easily, so the new cloned
> bio isn't necessary any more.
> 
> Not only fixes dm-zone's kernel panic, but also cleans up dm io
> accounting & split a bit.

You're conflating quite a few things here.  DM zone really has no
business accessing io->orig_bio (dm-zone.c can just as easily inspect
the tio->clone, because it hasn't been remapped yet it reflects the
io->origin_bio, so there is no need to look at io->orig_bio) -- but
yes I clearly broke things during the 5.18 merge and it needs fixing
ASAP.

But I'm (ab)using io->orig_bio assignment to indicate to completion
that it may proceed.  See these dm-5.19 commits to see it imposed even
further:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.19&id=311a8e6650601a79079000466db77386c5ec2abb
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.19&id=56219ebb5f5c84785aa821f755d545eae41bdb1a

And then leveraged here:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.19&id=4aa7a368370c2a172d5a0b8927c6332c4b6a3514

Could be all these dm-5.19 changes suck.. but I do know dm-zone.c is
too tightly coupled to DM core.  So I'll focus on that first, fix
5.18, and then circle back to "what's next?".

Mike