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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, 14 Aug 2015 15:23:48 -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
+ * 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 - This is a change of behavior from precise

- * Explanation - This patch checks to see if the source directory of a
+ * 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
+ * $ 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)
+ 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 not upstream, and will not be upstreamed *(see other info)

- * df command could stop working if I wrote the patch poorly. df might
+ * df command could stop working if I wrote the patch poorly. df might
not process mounts correctly.

[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 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.

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>

--
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https://bugs.launchpad.net/bugs/1432871

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

Status in coreutils package in Ubuntu:
New

Bug description:
[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
/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 not upstream, and will not be upstreamed *(see other info)

* df command could stop working if I wrote the patch poorly. df
might not process mounts correctly.

[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.

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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