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There are reports that since version 6.7 update-grub fails to find the
device of the root on systems without initrd and on a single device.

This looks like the device name changed in the output of
/proc/self/mountinfo:

6.5-rc5 working

  18 1 0:16 / / rw,noatime - btrfs /dev/sda8 ...

6.7 not working:

  17 1 0:15 / / rw,noatime - btrfs /dev/root ...

and "update-grub" shows this error:

  /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?)

This looks like it's related to the device name, but grub-probe
recognizes the "/dev/root" path and tries to find the underlying device.
However there's a special case for some filesystems, for btrfs in
particular.

The generic root device detection heuristic is not done and it all
relies on reading the device infos by a btrfs specific ioctl. This ioctl
returns the device name as it was saved at the time of device scan (in
this case it's /dev/root).

The change in 6.7 for temp_fsid to a...