Leftover mount in /etc/mtab pointing to /var/lib/ureadahead/debugfs/tracing after some runs of apt dist-upgrade followed by a reboot

[Grzegorz Ojrzanowski]

After a patching and a reboot cycle (possibly only the ones that involve running ureadahead triggers) a leftover mount entry is present in /etc/mtab pointing to /var/lib/ureadahead/debugfs/tracing

  root@localhost:~$ mount
  /dev/mapper/sysvg-root on / type ext4 (rw,errors=remount-ro)
  proc on /proc type proc (rw,noexec,nosuid,nodev)
  sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
  none on /sys/fs/cgroup type tmpfs (rw)
  none on /sys/fs/fuse/connections type fusectl (rw)
  none on /sys/kernel/debug type debugfs (rw)
  none on /sys/kernel/security type securityfs (rw)
  udev on /dev type devtmpfs (rw,mode=0755)
  devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
  tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
  none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
  none on /run/shm type tmpfs (rw,nosuid,nodev)
  none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)
  none on /sys/fs/pstore type pstore (rw)
  tracefs on /var/lib/ureadahead/debugfs/tracing type tracefs (rw,relatime)
  /dev/sda1 on /boot type ext4 (rw)
  systemd on /sys/fs/cgroup/systemd type cgroup (rw,noexec,nosuid,nodev,none,name=systemd)

  root@localhost:~$ mount | grep tracefs
  tracefs on /var/lib/ureadahead/debugfs/tracing type tracefs (rw,relatime)

But that mount point doesn't exist

  root@localhost:~$ ls -lha /var/lib/ureadahead/debugfs/
  total 8.0K
  drwxr-xr-x 2 root root 4.0K Jan 27 11:36 .
  drwxr-xr-x 3 root root 4.0K Jan 27 11:16 ..

That mtab entry can be removed by running 'umount /var/lib/ureadahead/debugfs/tracing', but only after creating that mount point first

  root@localhost:~$ mkdir /var/lib/ureadahead/debugfs/tracing

  root@localhost:~$ ls -lh /var/lib/ureadahead/debugfs/
  total 4.0K
  drwxr-xr-x 2 root root 4.0K Jan 27 11:28 tracing

  root@localhost:~$ umount /var/lib/ureadahead/debugfs/tracing
  umount: /var/lib/ureadahead/debugfs/tracing: not mounted

  root@localhost:~$ mount | grep tracefs

  root@localhost:~$ mount
  /dev/mapper/sysvg-root on / type ext4 (rw,errors=remount-ro)
  proc on /proc type proc (rw,noexec,nosuid,nodev)
  sysfs on /sys type sysfs (rw,noexec,nosuid,nodev)
  none on /sys/fs/cgroup type tmpfs (rw)
  none on /sys/fs/fuse/connections type fusectl (rw)
  none on /sys/kernel/debug type debugfs (rw)
  none on /sys/kernel/security type securityfs (rw)
  udev on /dev type devtmpfs (rw,mode=0755)
  devpts on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
  tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
  none on /run/lock type tmpfs (rw,noexec,nosuid,nodev,size=5242880)
  none on /run/shm type tmpfs (rw,nosuid,nodev)
  none on /run/user type tmpfs (rw,noexec,nosuid,nodev,size=104857600,mode=0755)
  none on /sys/fs/pstore type pstore (rw)
  /dev/sda1 on /boot type ext4 (rw)
  systemd on /sys/fs/cgroup/systemd type cgroup (rw,noexec,nosuid,nodev,none,name=systemd)

I'm experiencing this on several Ubuntu 14.04 servers, running on a VMware cluster.
Package versions worth noting are:
ureadahead 0.100.0-16
linux-image-4.4.0-59-generic 4.4.0-59.80~14.04.1
open-vm-tools 2:9.4.0-1280544-5ubuntu6.2

This also causes #1516451 and may be related to #499773

[Grzegorz Ojrzanowski (szponek)]

Just to note, 'mountall' also clears that mtab entry, but it also nukes contents of /tmp in the process, which might not be good if someone is running Passenger, which apparently keeps socket files in there (i.e. /tmp/passenger.1.0.xxx/generation-0/request )

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Grzegorz Ojrzanowski (szponek)]

Since it seems this bug manifests itself only after certain 'apt upgrade' runs that call ureadahead triggers, would backporting the fix from Vivid ureadahead package ver. 0.100.0-19 be an option to resolve it?
I'm thinking of the fix for https://bugs.launchpad.net/ubuntu/+source/ureadahead/+bug/777224 and the non-systemd change done to http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/vivid/ureadahead/vivid/revision/32#debian/rules