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

[Ming Lei]

On Tue, Apr 12, 2022 at 04:52:40PM -0400, Mike Snitzer wrote:
> 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.

You can just consider the cleanup part of this patches, :-)

1) no late assignment of ->orig_bio, and always set it in alloc_io()

2) no waiting on on ->origi_bio, especially the waiting is done in
fast path of dm_submit_bio_remap().

3) no split for io accounting


Thanks,
Ming