Subject: bug#17676: btrfs subvolumes and bind-mounts make df report incorrect and/or extra results

From: David Schleimer <[email-masked]>
Date: Tue, 3 Jun 2014 15:21:15 +0000

When you have btrfs mount where there is a both a subvolume of that mount, and the source of a bind-mount under that mount, df will report confusing results.  It will show results for the bind-mount instead of the main btrfs mount when you pass the subvoulme on the command line.  It may (depending on the system) show both the bind-mount and main mount in bare df results.

I've attached a script with the minimal repro instructions.  It needs to be run as root since it (temporarily) mounts filesystems under your tmpdir.

I tested on two machines:
Fedora 20 running kernel 3.14.4-200.fc20.x86_64, against both the system df 8.21 and df 8.22 built from source.

Centos 6.4 running a 3.10.39 variant, with significant backports, notably including most btrfs changes from mainline.  Testing against the system df 8.4, and df 8.22 again built from source.

On the fedora machine, /etc/mtab is a symlink to /proc/self/mounts.  We see output that refers to the correct block device in all cases.  However, we see a report for the bind-mount destination instead of the main btrfs mount when asking specifically for the subvolume path, and see results for both the main btrfs filesystem and the bind-mount when we run a bare df.

Transcript of Fedora output:

[root@caitsidhe dschleimer]# ./repro_instructions.sh
1024+0 records in
1024+0 records out
10485760 bytes (10 MB) copied, 0.00963354 s, 1.1 GB/s SMALL VOLUME: forcing mixed metadata/data groups

WARNING! - Btrfs v3.12 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

Turning ON incompat feature 'mixed-bg': mixed data and metadata block groups Turning ON incompat feature 'extref': increased hardlink limit per file to 65536 Created a data/metadata chunk of size 1048576 fs created label (null) on block
        nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MiB Btrfs v3.12 Create subvolume 'filesystem/subvolume'

Begin unexpected output
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/loop0         10240    36      6112   1% /tmp/tmp.biUryZcwD9/binddest
Expected report for the main btrfs mount, not the bind-mount
/dev/loop0              btrfs        10240       36      6112   1% /tmp/tmp.biUryZcwD9/filesystem
/dev/loop0              btrfs        10240       36      6112   1% /tmp/tmp.biUryZcwD9/binddest
Expected only one btrfs filesystem
/home/dschleimer
[root@caitsidhe dschleimer]# PATH=/home/dschleimer/Downloads/coreutils-8.22/src/:$PATH ./repro_instructions.sh
1024+0 records in
1024+0 records out
10485760 bytes (10 MB) copied, 0.012752 s, 822 MB/s SMALL VOLUME: forcing mixed metadata/data groups

WARNING! - Btrfs v3.12 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

Turning ON incompat feature 'mixed-bg': mixed data and metadata block groups Turning ON incompat feature 'extref': increased hardlink limit per file to 65536 Created a data/metadata chunk of size 1048576 fs created label (null) on block
        nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MiB Btrfs v3.12 Create subvolume 'filesystem/subvolume'

Begin unexpected output
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/loop0         10240    36      6112   1% /tmp/tmp.TEBR2D9Vta/binddest
Expected report for the main btrfs mount, not the bind-mount
/dev/loop0              btrfs        10240       36      6112   1% /tmp/tmp.TEBR2D9Vta/filesystem
/dev/loop0              btrfs        10240       36      6112   1% /tmp/tmp.TEBR2D9Vta/binddest
Expected only one btrfs filesystem
/home/dschleimer

On Centos, /etc/mtab is a plain file.  When asking specifically for the subvolume, we see a report for the bind-mount dest which lists the bind-mount source as the filesystem.  When we run a bare df, we see only one record with the file backing the loopback device as the filesystem, but the bind-mount as the mount-point.

Centos transcript:

[08:03:09 Tue Jun 03 2014] [email-masked] /home/dschleimer dschleimer  282 $ ./repro_instructions.sh
1024+0 records in
1024+0 records out
10485760 bytes (10 MB) copied, 0.0101568 s, 1.0 GB/s

WARNING! - Btrfs v0.20-rc1-358-g194aa4a IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using

SMALL VOLUME: forcing mixed metadata/data groups Created a data/metadata chunk of size 1048576 fs created label (null) on block
        nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MB Btrfs v0.20-rc1-358-g194aa4a Create subvolume 'filesystem/subvolume'

Begin unexpected output
Filesystem           1K-blocks      Used Available Use% Mounted on
/tmp/tmp.V4FNWcBXke/filesystem/subvolume/bindsource
                         10240        36      6112   1% /tmp/tmp.V4FNWcBXke/binddest
Expected report for the main btrfs mount, not the bind-mount
df: `/mnt/gvfs': Function not implemented /tmp/tmp.V4FNWcBXke/block
             btrfs       10240        36      6112   1% /tmp/tmp.V4FNWcBXke/filesystem
Expected only one btrfs filesystem
/home/dschleimer

[08:09:39 Tue Jun 03 2014] [email-masked] /home/dschleimer dschleimer  282 $ PATH=/data/users/dschleimer/coreutils-8.22/src/:$PATH ./repro_instructions.sh
1024+0 records in
1024+0 records out
10485760 bytes (10 MB) copied, 0.0116229 s, 902 MB/s

WARNING! - Btrfs v0.20-rc1-358-g194aa4a IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using

SMALL VOLUME: forcing mixed metadata/data groups Created a data/metadata chunk of size 1048576 fs created label (null) on block
        nodesize 4096 leafsize 4096 sectorsize 4096 size 10.00MB Btrfs v0.20-rc1-358-g194aa4a Create subvolume 'filesystem/subvolume'

Begin unexpected output
Filesystem                                          1K-blocks  Used Available Use% Mounted on
/tmp/tmp.o7w6NcN178/filesystem/subvolume/bindsource     10240    36      6112   1% /tmp/tmp.o7w6NcN178/binddest
Expected report for the main btrfs mount, not the bind-mount
/tmp/tmp.o7w6NcN178/block                        btrfs      10240         36       6112   1% /tmp/tmp.o7w6NcN178/filesystem
Expected only one btrfs filesystem
/home/dschleimer