Comment 19 for bug 1559723

cat /proc/mounts
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
udev /dev devtmpfs rw,relatime,size=8157548k,nr_inodes=2039387,mode=755 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
tmpfs /run tmpfs rw,nosuid,noexec,relatime,size=1634416k,mode=755 0 0
/dev/mapper/md0_crypt / btrfs ro,relatime,space_cache,subvolid=256,subvol=/@ 0 0
none /sys/fs/cgroup tmpfs rw,relatime,size=4k,mode=755 0 0
none /sys/fs/fuse/connections fusectl rw,relatime 0 0
none /sys/kernel/debug debugfs rw,relatime 0 0
none /sys/kernel/security securityfs rw,relatime 0 0
none /tmp tmpfs rw,relatime,size=6291456k 0 0
none /run/lock tmpfs rw,nosuid,nodev,noexec,relatime,size=5120k 0 0
none /run/shm tmpfs rw,nosuid,nodev,relatime 0 0
none /run/user tmpfs rw,nosuid,nodev,noexec,relatime,size=102400k,mode=755 0 0
none /sys/fs/pstore pstore rw,relatime 0 0
none /var/tmp/mysql tmpfs rw,relatime,size=6291456k 0 0
/dev/mapper/md0_crypt /home btrfs ro,relatime,space_cache,subvolid=258,subvol=/@home 0 0
/dev/mapper/md0_crypt /backup btrfs ro,relatime,space_cache,subvolid=263,subvol=/@backup 0 0
/dev/sda2 /boot ext4 rw,relatime,stripe=4,data=ordered 0 0
/dev/sdb2 /boot2 ext4 rw,relatime,stripe=4,data=ordered 0 0
systemd /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,name=systemd 0 0
/dev/mapper/md0_crypt /mnt/x btrfs ro,relatime,space_cache,subvolid=5,subvol=/ 0 0

All the btrfs mount points were mounted read-only (they all originate from one partition /dev/mapper/md0_crypt). I started the btrfs balance online (some days ago), meaning I mounted /dev/mapper/md0_crypt to mount point /mnt/x for the btrfs balance operation.