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[Bug 1432871] Re: `df` shows bind mounts instead of real mounts.

To: touch-packages@xxxxxxxxxxxxxxxxxxx
From: Dave Chiluk <1432871@xxxxxxxxxxxxxxxxxx>
Date: Fri, 02 Oct 2015 22:19:50 -0000
Reply-to: Bug 1432871 <1432871@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx

** Description changed:

[Impact]

* df displays bind mounts instead of "real" mounts if the bind mount is
mounted to a shorter directory.

* justification - This is a change of behavior from precise

* Explanation - This patch checks to see if the source directory of a
mount is contains the destination directory of the mount it's replacing
*(check if it's bind mount from the original directory).

[Test Case]

* $ mount
<snip>
- 192.168.1.2:/raid on /raid type nfs
+ 192.168.1.2:/raid on /raid type nfs
/dev/sdc5 on /data type ext4 (rw)
<snip>
/data/a on /a type none (rw,bind)
/raid/temp on /b type none (rw,bind)

$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/data/a 449830616 229975284 196982196 54% /a
/raid/temp 7752072192 5581343744 1780023296 76% /b
</snip>

I'd expect to see the real mount prioritized over the bind mount. Like so.
$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/dev/sdc5 449830616 229975284 196982196 54% /data
192.168.1.2:/raid 7752072192 5581438976 1779929088 76% /raid
<snip>

[Regression Potential]

- * Patch is not upstream, and will not be upstreamed *(see other info)
+ * Patch is upstreamed.

- * df command could stop working if I wrote the patch poorly. df might
- not process mounts correctly.
+ * df command now relies on /proc/self/mountinfo

[Other Info]

- * The behavior of df, mount and similar number of other commands has changed going forward. Previously these commands all processed /etc/mtab which was maintained by the mount command. Going forward they still process /etc/mtab, but this is simply a symlink to /proc/mounts now which is maintained by the kernel and likely significantly more accurate. Unlike the mount command, the kernel makes no distinction between bind mounts and normal mounts. This is evident by the fact that you can mount a device, bind mount from that device, and then unmount the original mount. The default behavior of df in this case is to simply pick the mounted directory for a device that is the shortest since it has no other information to go on from /proc/mounts.
- * Theoretically df, mount and a number of other commands could be modified to run ioctl commands to determine if the mount is from a root inode of the mounted filesystem to determine preference, but that is really more of an upstream exercise.
+ * The behavior of df, mount and similar number of other commands has
+ changed going forward. Previously these commands all processed
+ /etc/mtab which was maintained by the mount command. Going forward they
+ still process /etc/mtab, but this is simply a symlink to /proc/mounts
+ now which is maintained by the kernel and slightly more accurate.
+ Unlike the mount command, the kernel makes no distinction between bind
+ mounts and normal mounts. This is evident by the fact that you can
+ mount a device, bind mount from that device, and then unmount the
+ original mount. The default behavior of df in this case is to simply
+ pick the mounted directory for a device that is the shortest since it
+ has no other information to go on from /proc/mounts. Moving to using
+ /proc/self/mountinfo resolves this issue, and is what upstream is doing
+ moving forward.

Original bug.
________________________________________________________________________
Depending on mount path length df sometimes prioritizes showing bind mounts over real mounts

for example.
$ mount
<snip>
192.168.1.2:/raid on /raid type nfs (rw,nosuid,nodev,noexec,vers=4,addr=192.168.1.2,clientaddr=192.168.1.3)
/dev/sdc5 on /data type ext4 (rw)
<snip>
/data/a on /a type none (rw,bind)
/raid/temp on /b type none (rw,bind)

$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/data/a 449830616 229975284 196982196 54% /a
/raid/temp 7752072192 5581343744 1780023296 76% /b
</snip>

** Description changed:

[Impact]

* df displays bind mounts instead of "real" mounts if the bind mount is
mounted to a shorter directory.

- * justification - This is a change of behavior from precise
+ * justification - When trusty moved to using /proc/mounts this changed
+ behavior from precise. Additionally it doesn't make sense that a bind
+ mount should show up in df over a real root mount.

- * Explanation - This patch checks to see if the source directory of a
- mount is contains the destination directory of the mount it's replacing
- *(check if it's bind mount from the original directory).
+ * Explanation - These patches change behavior of df to rely on
+ /proc/self/mountinfo which has more complete info than /proc/mounts.
+ Such as what directory of the filesystem was used as the source of the
+ mount. Additionally given this new information there is a patch on df
+ itself to make use of this new information.

[Test Case]

* $ mount
<snip>
192.168.1.2:/raid on /raid type nfs
/dev/sdc5 on /data type ext4 (rw)
<snip>
/data/a on /a type none (rw,bind)
/raid/temp on /b type none (rw,bind)

$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/data/a 449830616 229975284 196982196 54% /a
/raid/temp 7752072192 5581343744 1780023296 76% /b
</snip>

[Regression Potential]

* Patch is upstreamed.

* df command now relies on /proc/self/mountinfo

[Other Info]

* The behavior of df, mount and similar number of other commands has
changed going forward. Previously these commands all processed
/etc/mtab which was maintained by the mount command. Going forward they
still process /etc/mtab, but this is simply a symlink to /proc/mounts
now which is maintained by the kernel and slightly more accurate.
Unlike the mount command, the kernel makes no distinction between bind
mounts and normal mounts. This is evident by the fact that you can
mount a device, bind mount from that device, and then unmount the
original mount. The default behavior of df in this case is to simply
pick the mounted directory for a device that is the shortest since it
has no other information to go on from /proc/mounts. Moving to using
/proc/self/mountinfo resolves this issue, and is what upstream is doing
moving forward.

Original bug.
________________________________________________________________________
Depending on mount path length df sometimes prioritizes showing bind mounts over real mounts

$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/data/a 449830616 229975284 196982196 54% /a
/raid/temp 7752072192 5581343744 1780023296 76% /b
</snip>

--
You received this bug notification because you are a member of Ubuntu
Touch seeded packages, which is subscribed to coreutils in Ubuntu.
https://bugs.launchpad.net/bugs/1432871

Title:
`df` shows bind mounts instead of real mounts.

Status in coreutils package in Ubuntu:
Incomplete

Bug description:
[Impact]

* df displays bind mounts instead of "real" mounts if the bind mount
is mounted to a shorter directory.

* justification - When trusty moved to using /proc/mounts this
changed behavior from precise. Additionally it doesn't make sense
that a bind mount should show up in df over a real root mount.

* Explanation - These patches change behavior of df to rely on
/proc/self/mountinfo which has more complete info than /proc/mounts.
Such as what directory of the filesystem was used as the source of the
mount. Additionally given this new information there is a patch on df
itself to make use of this new information.

[Test Case]

* $ mount
<snip>
192.168.1.2:/raid on /raid type nfs
/dev/sdc5 on /data type ext4 (rw)
<snip>
/data/a on /a type none (rw,bind)
/raid/temp on /b type none (rw,bind)

$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/data/a 449830616 229975284 196982196 54% /a
/raid/temp 7752072192 5581343744 1780023296 76% /b
</snip>

[Regression Potential]

* Patch is upstreamed.

* df command now relies on /proc/self/mountinfo

[Other Info]

* The behavior of df, mount and similar number of other commands has
changed going forward. Previously these commands all processed
/etc/mtab which was maintained by the mount command. Going forward
they still process /etc/mtab, but this is simply a symlink to
/proc/mounts now which is maintained by the kernel and slightly more
accurate. Unlike the mount command, the kernel makes no distinction
between bind mounts and normal mounts. This is evident by the fact
that you can mount a device, bind mount from that device, and then
unmount the original mount. The default behavior of df in this case
is to simply pick the mounted directory for a device that is the
shortest since it has no other information to go on from /proc/mounts.
Moving to using /proc/self/mountinfo resolves this issue, and is what
upstream is doing moving forward.

Original bug.
________________________________________________________________________
Depending on mount path length df sometimes prioritizes showing bind mounts over real mounts

$df
Filesystem 1K-blocks Used Available Use% Mounted on
<snip>
/data/a 449830616 229975284 196982196 54% /a
/raid/temp 7752072192 5581343744 1780023296 76% /b
</snip>

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