[PATCH] btrfs: Use default subvolume as filesystem root

[Matthias Brugger]

On 01/09/2021 13:36, Tom Rini wrote:
> 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!
> 

It's a bugfix, as loading files from BTRFS, at least in openSUSE does not work
without it.

Thanks for the quick answer!
Matthias