Re: [PATCH 2/2] btrfs: fix and document the zoned device choice in alloc_new_bio

[David Sterba <dsterba@suse.cz>]

On Fri, Mar 25, 2022 at 09:09:56AM +0000, Johannes Thumshirn wrote:
> On 24/03/2022 17:54, Christoph Hellwig wrote:
> > Zone Append bios only need a valid block device in struct bio, but
> > not the device in the btrfs_bio.  Use the information from
> > btrfs_zoned_get_device to set up bi_bdev and fix zoned writes on
> > multi-device file system with non-homogeneous capabilities and remove
> > the pointless btrfs_bio.device assignment.
> > 
> > Add big fat comments explaining what is going on here.
> 
> Looks like the old code worked by sheer luck, as we had wbc set and thus
> always assigned fs_info->fs_devices->latest_dev->bdev to the bio. Which 
> would obviously not work on a multi device FS.

No, it worked fine because the real bio is set just before writing the
data somewhere deep in the io submit path in submit_stripe_bio().

That it has to be set here is because of the cgroup implementation that
accesses it, see 429aebc0a9a0 ("btrfs: get bdev directly from fs_devices
in submit_extent_page").

Which brings me to the question if Christoph's fix is correct because
the comment for the wbc + zoned append is assuming something that's not
true.