[Bug 499773] Re: Race with ureadahead can mean that /var/lib/ureadahead/debugfs appears in /etc/mtab

[T.J. Crowder]

Help me out here, folks: Is this harmless or not? On the one hand, if
it's just a matter of /etc/mtab claiming there's something mounted that
isn't, that seems (relatively) harmless. On the other hand, we have at
least one user reporting here (comment #9) that they see a significant
performance impact when this happens, and multiple users reporting that
it causes incorrect file system full errors (although I'm having trouble
seeing why it would do that).

So:

* Does this really have a performance impact, or is that user mistaken?

* Does this really cause false "full" errors, or are those users
mistaken?

10.04 is an LTS release. I'm seeing this on my fully-updated 10.04
server, which surprises me nearly two years later if it really has any
impacts.

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Title:
  Race with ureadahead can mean that /var/lib/ureadahead/debugfs appears
  in /etc/mtab

Status in “mountall” package in Ubuntu:
  Triaged

Bug description:
  When the system is booted, the mountall process may notice a
  temporarily-mounted file system at /var/lib/ureadahead/debugfs and
  record that fact in /etc/mtab.  Later in the boot process, this file
  system is unmounted, but mountall doesn't record unmounts, so it never
  "forgets" this path when it's unmounted.

  Mountall should if necessary remove this mount from mtab.

  To confirm on your system that it is indeed not really mounted, look for it in /proc/self/mountinfo
  Otherwise this may be due to a crash of the ureadahead process during boot.

  ---- original report ----

  Binary package hint: ureadahead

  $ mount
  /dev/sda1 on / type ext3 (rw,relatime,errors=remount-ro)
  proc on /proc type proc (rw)
  none on /sys type sysfs (rw,noexec,nosuid,nodev)
  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 tmpfs (rw,mode=0755)
  none on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
  none on /dev/shm type tmpfs (rw,nosuid,nodev)
  none on /var/run type tmpfs (rw,nosuid,mode=0755)
  none on /var/lock type tmpfs (rw,noexec,nosuid,nodev)
  none on /lib/init/rw type tmpfs (rw,nosuid,mode=0755)
  none on /var/lib/ureadahead/debugfs type debugfs (rw)
  /dev/sda6 on /home type ext4 (rw,relatime)
  binfmt_misc on /proc/sys/fs/binfmt_misc type binfmt_misc (rw,noexec,nosuid,nodev)
  gvfs-fuse-daemon on /home/hildeb/.gvfs type fuse.gvfs-fuse-daemon (rw,nosuid,nodev,user=hildeb)

  /var/lib/ureadahead/debugfs is left lingering around. It's use is
  somewhat limited, since most Ubuntu users are not kernel developers --
  if it's used as intended: http://kerneltrap.org/node/4394

  ProblemType: Bug
  Architecture: amd64
  Date: Wed Dec 23 11:04:11 2009
  DistroRelease: Ubuntu 9.10
  NonfreeKernelModules: nvidia
  Package: ureadahead 0.90.3-2
  ProcEnviron:
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  ProcVersionSignature: Ubuntu 2.6.31-17.54-generic
  SourcePackage: ureadahead
  Uname: Linux 2.6.31-17-generic x86_64

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