[PATCH] btrfs: Use default subvolume as filesystem root

[Tom Rini]

On Wed, Sep 01, 2021 at 01:28:30PM +0200, Matthias Brugger wrote:
> Hi Tom,
> 
> On 02/08/2021 01:06, Qu Wenruo wrote:
> > 
> > 
> > On 2021/8/2 上午4:52, Matwey V. Kornilov wrote:
> >> BTRFS volume consists of a number of subvolumes which can be mounted separately
> >> from each other. The top-level subvolume always exists even if no subvolumes
> >> were created manually. A subvolume can be denoted as the default subvolume i.e.
> >> the subvolume which is mounted by default.
> >>
> >> The default "default subvolume" is the top-level one, but this is far from the
> >> common practices used in the wild. For instance, openSUSE provides an OS
> >> snapshot/rollback feature based on BTRFS. To achieve this, the actual OS root
> >> filesystem is located into a separate subvolume which is "default" but not
> >> "top-level". That means that the /boot/dtb/ directory is also located inside
> >> this default subvolume instead of top-level one.
> >>
> >> However, the existing btrfs u-boot driver always uses the top-level subvolume
> >> as the filesystem root. This behaviour 1) is inconsistent with
> >>
> >>      mount /dev/sda1 /target
> >>
> >> command, which mount the default subvolume 2) leads to the issues when
> >> /boot/dtb cannot be found properly (see the reference).
> > 
> > I also noticed the problem in the past, but forgot to fix it....
> > 
> >>
> >> This patch uses the default subvolume as the filesystem root to overcome
> >> mentioned issues.
> >>
> >> Reference: https://bugzilla.suse.com/show_bug.cgi?id=1185656
> >> Signed-off-by: Matwey V. Kornilov <matwey.kornilov at gmail.com>
> > 
> > Reviewed-by: Qu Wenruo <wqu at suse.com>
> > 
> 
> I can see that this patch is marked in your patchwork queue as "Need Review /
> ACK". Qu is one of our core btrfs developer who reviewed the patch. Apart from
> that we have it running on openSUSE on top of v2021.07 for some time without any
> issues.

Ah, yup.  Qu is one of the people I do look for to have reviewed a btrfs
patch before I apply it (and I throw things under Need Review / ACK as a
note-to-self to make sure a patch does have one, when I can expect one,
before applying, FWIW).

> Would it be possible to merge this for v2021.10 or do you see any blocker here?

I think I had mentally filed it was feature not bugfix and was going to
hold off, but since you're asking, yes, I can grab it for this release.
Thanks!

-- 
Tom
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