[SOLVED] How to pass permanent rootflags to snapshots' kernel?

[FGD-Garuda]

Hello,

What is the right way to pass permanent rootflags to the snapshots?

So far I have in my /etc/default/grub config this line:

GRUB_CMDLINE_LINUX_DEFAULT="rw quiet splash showopts rootflags=noatime"

Then in /boot/grub/grub-btrfs.cfg I have:

linux "/@snapshots/daily/root2020-12-27_19H05/boot/vmlinuz-5.8.0-33-generic" root=/dev/sda1 rw rw quiet splash showopts rootflags=noatime rootflags=subvol="@snapshots/daily/root2020-12-27_19H05"

Don't mind the duplication of rw, this one is passed fine once.
However you can see 2 rootflags=, one with my mount option and the other with the subvol coming from /etc/grub.d/41_snapshot-btrfs:

linux \"${boot_dir_root_grub}/"${k}"\" root="${LINUX_ROOT_DEVICE}" rw ${kernel_parameters} rootflags=subvol=\""${snap_dir_name}"\""

That above line explains the duplication of rw but also the addition of a second rootflags=.

When I boot from that snapshot, findmnt -l tells me the snapshot has been mounted with default relatime. Which means having twice rootflags= does not work, so then which way can I pass permanent rootflags to the snapshots?

Should I simply edit 41_snapshots-btrfs and add ,noatime after rootflags=subvol=\""${snap_dir_name}"\""?
And remove the rw at the same time.

That would probably work but I'm not sure it's the cleanest way of doing it. That would mean some parms can be written in the GRUB_CMDLINE_LINUX_DEFAULT line but some others can't and you have to go elsewhere to do it. Maybe that's how it is, maybe there is a good valid reason for that too, I don't know, that's why I'm asking. :)

Thanks

[Antynea]

Hello,

That would probably work but I'm not sure it's the cleanest way of doing it. That would mean some parms can be written in the GRUB_CMDLINE_LINUX_DEFAULT line but some others can't and you have to go elsewhere to do it. Maybe that's how it is, maybe there is a good valid reason for that too, I don't know, that's why I'm asking. :)

This is a weakness of the script, which needs to be improved.

The duplication is normal, It is always the last parameter that is used for kernel parameters.
The parameter "rw" is a residue of an old version of the script.
rootflags=subvol=\""${snap_dir_name}"\"" was placed at the end, because some users already specified this parameter in their /etc/default/grub configuration file and this was a problem.

While waiting for this part of the script to be rewritten, you can proceed as you described:
Edit 41_snapshots-btrfs
remove rw parameter,
modified the rootflags parameter by adding your options , this should be correct (not tested)
rootflags=subvol=\""${snap_dir_name}"\"",noatime

[Kr1ss-XD]

That sounds nice, but there are no boot parameters or mount options for / defined in fstab, unless I'm missing something.

Basically, it contains mount rules for partitions/subvolumes other than /. (If there's an fstab entry for /, it doesn't have any effect.) Hence I'd say it would be confusing if users could define rootflags at this place...

EDIT/ Never mind, seems like if a / mountpoint is defined in that file, the filesystem root will be remounted with these options during the boot process. My bad.

[Antynea]

That sounds nice, but there are no boot parameters or mount options for / defined in fstab, unless I'm missing something.

Systemd reads the /etc/fstab file.
if the line for / is present and match , it remount / with the indicated options.

cat /etc/fstab

UUID=bbad010b-a5d4-41a5-968f-ff9a609e93e3       /               btrfs           rw,relatime,space_cache,subvolid=257,subvol=/@racine     0 0

findmnt

/                                     /dev/sda1[/@racine]    btrfs      rw,relatime,space_cache,subvolid=257,subvol=/@racine

Okay, you edited your post during my reply :)

[Antynea]

So I'd like to retrieve the mount options in /etc/fstab and apply them to the snapshot. Then also allow an option to customize this variable.

[Kr1ss-XD]

Yeah that's cool. 👍 I think I could implement that if you like.

[Antynea]

treat yourself :)