can not reinstall grub after windows install: btrfs subvolumes

Question

(To clarify: this is about fixing grub by using a LiveCD to rebuild grub, a pretty standard operation e.g. https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows, with the difficulty being btrfs in use)

Ubuntu partition is /dev/sda5 and is btrfs, with encrypted home.

When I

mount /dev/sda5 /mnt
cd /mnt 
ls

I get two directories: @ and @home and the / file system is under @

chroot /mnt

fails with 'failed to run command /bin/bash'

chroot /mnt/@

looks like it works, since it lets me mount the /proc /dev etc via

for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done

but update-grub fails with

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

fstab looks like this:

UUID=<...> / btrfs defaults,subvol=@ 0 1

swap and home are encrypted.

Answer 1

Ah, the trick with the subvolume is to mount the partition like this:

sudo mount -t btrfs -o subvol=@ /dev/sda5 /mnt 

update-grub succeeds.

I needed to do these steps as well (via How can I repair grub? (How to get Ubuntu back after installing Windows?))

grub-install /dev/sda
update-grub # In order to find and add windows to grub menu.