[Bug 1968135] Re: BTRFS root subvolume @ mounted always as readonly (... ro rootflags=subvol=@ ...) on Jammy beta

[Milan Ulej]

The ro flag should not be on booted system, unfortunately this bug
passed to the release. This caused jammy LTS unusable if rootfs is on
BTRFS (without workaround above).

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https://bugs.launchpad.net/bugs/1968135

Title:
  BTRFS root subvolume @ mounted always as readonly (... ro
  rootflags=subvol=@ ...) on Jammy beta

Status in systemd package in Ubuntu:
  New

Bug description:
  = Description =
  On Ubuntu Jammy Jellyfish (beta 220330) where root (/) is installed on single BTRFS formated partition (no LVM, no separate /boot), then root subvolume (subvol=@) is always mounted as read only. There is a kernel flag (ro) appended to GRUB configuration file /boot/grub/grub.cfg created by script /usr/sbin/update-grub during every kernel update. Only home subvolume (@home) remains mounted as rw according to fstab which is correct. Read-only mounted root (/) leads to many issues.
  Image: Jammy Beta (220330)

  = Steps to Reproduce =
  1. Install Jammy Beta (220330) on EFI PC x86_64 GPT partitioning and select FS root (/) to one single BTRFS partition (where subvolumes @ and @home are created). There are only 3 partitions EFI system partition (/dev/sda1), one swap (sda2), and one BTRFS formated (/dev/sda3) for FS root (/).
  2. Launch the kernel via the default grub entry (or look for kernel boot parameters pressing <e>) - root subvolume (@) is mounted as readonly.

  = Expected Results =
  root / should be mounted as rw (mount -v | grep @), there should be no (ro) flag in /proc/cmdline for FS root. It is possible to temporarily avoid this behaviour in /etc/defaul/grub:
  GRUB_CMDLINE_LINUX_DEFAULT="rw quiet splash"
  Then there still remains "ro" kernel parameter which is followed by "rw quiet splash" at the end.
   
  = Actual Results =
  in grub.cfg:
  linux /@/boot/vmlinuz-5.15.0-23-generic root=UUID=xxxx-xxxx-... rootflags=subvol=@ ro quiet splash $vt_handoff
  in /proc/cmdline:
  BOOT_IMAGE=/@/boot/vmlinuz-5.15.0-25-generic root=UUID=981dd6de-02fc-41a9-9e2a-09dbbc776b2b ro rootflags=subvol=@ quiet splash vt.handoff=7

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