Starting docker.io remounts root filesystem read-only

[Jouni K. Seppänen]

I'm on Ubuntu 14.04.3 LTS, and recently upgraded to docker.io 1.6.2~dfsg1-1ubuntu4~14.04.1. This package provides the file /etc/init/docker.conf, which includes this:

pre-start script
        # Cleanup any stale mounts left from previous shutdown
        # see https://bugs.launchpad.net/ubuntu/+source/docker.io/+bug/1404300
       grep "mapper/docker" /proc/mounts | awk '{ print $2 }' | \
               xargs -r umount || true
end script

My host's root filesystem is on an LVM volume named "docker1-root", and so /proc/mounts includes the line

/dev/mapper/docker1-root / ext4 rw,relatime,errors=remount-ro,data=ordered 0 0

Thus running the docker.io pre-start script causes the root filesystem to be remounted read-only.

[Michael Hudson-Doyle (mwhudson)]

Looks like a real problem, no idea of priority though.