Re: booting btrfs

[Chris Murphy]

On Oct 14, 2013, at 5:09 PM, Chris Murphy <address@hidden> wrote:

> 
> On Oct 14, 2013, at 3:01 PM, Vladimir 'φ-coder/phcoder' Serbinenko 
> <address@hidden> wrote:
> 
>> On 14.10.2013 22:45, Chris Murphy wrote:
>>> 
>>> On Oct 14, 2013, at 1:29 PM, Vladimir 'φ-coder/phcoder' Serbinenko 
>>> <address@hidden> wrote:
>>> 
>>>>> So it seems that GRUB is using relative pathnames to the default 
>>>>> subvolume.
>>>> This is not intentional. When this part of code was written there was no
>>>> set-default available at all so this couldn't be tested and I simply
>>>> followed the specification. It told to take root_tree and
>>>> root_dir_objectid from superblock then go to "default" directory. What
>>>> of this needs to be changed? Just remove "default" and make it part of
>>>> path? We would need to change grub-mkrelpath to match runtime behaviour.
>>>> Is there a way to detect that mountinfo gives garbage and somehow get
>>>> where the real root points?
>>> 
>>> Here's the response. It seems similar but not identical to what you 
>>> described above.
>>> 
>>> http://www.mail-archive.com/address@hidden/msg27955.html
>>> 
>> Possibly this is the answer:
>> === modified file 'grub-core/fs/btrfs.c'
>> --- grub-core/fs/btrfs.c     2013-01-21 01:33:46 +0000
>> +++ grub-core/fs/btrfs.c     2013-10-14 21:00:53 +0000
>> @@ -1217,7 +1217,7 @@
>> 
>>  *type = GRUB_BTRFS_DIR_ITEM_TYPE_DIRECTORY;
>>  *tree = data->sblock.root_tree;
>> -  key->object_id = data->sblock.root_dir_objectid;
>> +  key->object_id = grub_cpu_to_le64_compile_time (5);
>>  key->type = GRUB_BTRFS_ITEM_TYPE_DIR_ITEM;
>>  key->offset = 0;
>>  skip_default = 1;
>> 
> 
> I applied this to current bzr and built it. With the default default 
> subvolume, i.e set-default is ID 5, I get:
> 
> GRUB loading ..
> Welcome to GRUB!
> 
> error: file '/' not found.
> Entering rescue mode…
> grub rescue>
> 


Nevermind. The /etc/fstab was technically not valid at the time I ran 
grub-install, because I was using subvolid= there, and grub-install probably 
picked that up.

So with your change applied, I can still boot with 'btrfs subvol set-default 5 
/'.  However if I change the default subvolume, I get the same results as 
without the change:

error: file 'boot/grub/i386-pc/normal.mod' not found
Entering rescue mode…

grub rescue> set
prefix=(hd0,msdos1)/boot/grub
root=hd0,msdos1
grub rescue> ls (hd0,msdos1)/
youfoundme


So instead of it listing the subvolumes in the top level ID 5 subvolume, which 
includes /boot, it lists 'youfoundme' which is a file in a nested subvolume 
with ID 266 that has been 'btrfs subvol set-default 266 /'


Chris Murphy