Petitboot has no awareness of BTRFS subvolumes (and can't boot OSs installed into one)

[nroach44]

When installing Debian 12, if btrfs is chosen for the rootfs, the installer places the whole system in a subvolume called @rootfs.

If there is no /boot volume configured, the boot files end up in /<mountpoint>/@rootfs/boot/....

GRUB2 handles this fine (on EFI at least), but petitboot doesn't find the config, so it can't boot the system.

Placing an appropriate config file in the root of the top-level "subvolume" allows a work-around, but naturally that requires preparation.

Perhaps, either all subvolumes should be mounted, or a specific list of subvolumes, if found, should be mounted (and treated as separate devices).

[nroach44]

Just to update for anyone else that stumbles upon this:

Petitboot does know to check for a subvolume, but it only looks for one:

https://github.com/open-power/petitboot/blob/master/discover/device-handler.c#L2075

	/* On btrfs a device's root may be under a subvolume path */
	path = join_paths(dev, dev->mount_path, "@");

If you change @ to @rootfs it'll pick up the default debian root subvolume fine.

[jk-ozlabs]

Thanks for the info - I have almost zero experience with btrfs, so will have to do some digging to see what the expected behaviour would be.

The difference here is just the naming of that subvolume, right? Should this be something autodetected, or is GRUB getting the names from elsewhere?

[nroach44]

I can't say for certain in the case of an MBR install, but an UEFI install (of debian) puts a minimal grub.cfg (with the subvolume ID inside) in the ESP:

sudo ls -la /boot/efi/EFI/debian
total 5980
drwx------ 2 root root    4096 Mar 17 09:59 .
drwx------ 4 root root    4096 Mar 17 19:16 ..
-rwx------ 1 root root     108 Mar 29 14:43 BOOTX64.CSV
-rwx------ 1 root root   87376 Mar 29 14:43 fbx64.efi
-rwx------ 1 root root     196 Mar 29 14:43 grub.cfg
-rwx------ 1 root root 4199872 Mar 29 14:43 grubx64.efi
-rwx------ 1 root root  850176 Mar 29 14:43 mmx64.efi
-rwx------ 1 root root  960080 Mar 29 14:43 shimx64.efi

search.fs_uuid 56d5db08-f01c-4057-964e-8c8224df7e3c root hd0,gpt2 
set prefix=($root)'/@rootfs/boot/grub'
configfile $prefix/grub.cfg

So perhaps that's a good spot to look in addition to the current places.

In regards to BTRFS directly, I think the most reasonable outcome is to either

Mount the root volume (which is already done), then look at the list of subvolumes and their locations, and scan those locations directly. (Subvolumes exist as directories in the root volume):

sudo btrfs subvolume list /
ID 256 gen 246546 top level 5 path @rootfs
ID 257 gen 30751 top level 256 path var/lib/portables
ID 258 gen 30751 top level 256 path var/lib/machines
ID 259 gen 246546 top level 5 path .beeshome
ID 260 gen 233739 top level 256 path srv/vmstorage
ID 261 gen 233739 top level 256 path srv/software

or

Just specifically gather a list of the default subvolume names / locations from distributions and check those, if they exist, for boot config files.

[VenomousSteam81]

why not add a config option to the petitboot config file? something like rootfs or root
not sure how installing this works, but maybe prompt the user on install to ask for this variable as well?