confusing mountinfo output when bind-mounting files
Hi all,
I'm seeing some strange behavior when bind mounting files from a btrfs subvolume. Consider the output below:
root@criu2:/tmp# mount -o loop /tmp/tester.btrfs /tmp/dir1 root@criu2:/tmp# touch dir1/file root@criu2:/tmp# sudo mount --bind dir1/file dir2/file root@criu2:/tmp# grep "/tmp/dir" /proc/self/mountinfo 128 24 0:45 / /tmp/dir1 rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/ 129 24 0:45 /file /tmp/dir2/file rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/file root@criu2:/tmp# btrfs --version btrfs-progs v4.4 root@criu2:/tmp# uname -a Linux criu2 4.4.0-8-generic #23-Ubuntu SMP Wed Feb 24 20:45:30 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
The issue here is that the "subvol=" mount option for the target of the bind mount is "/file" when no such subvolume actually exists. Is this intended? It's confusing to say the least, but seems like a bug to me.
Tycho
Quoted 16 lines · Tycho Andersen
Hi all,
I'm seeing some strange behavior when bind mounting files from a btrfs subvolume. Consider the output below:
root@criu2:/tmp# mount -o loop /tmp/tester.btrfs /tmp/dir1 root@criu2:/tmp# touch dir1/file root@criu2:/tmp# sudo mount --bind dir1/file dir2/file root@criu2:/tmp# grep "/tmp/dir" /proc/self/mountinfo 128 24 0:45 / /tmp/dir1 rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/ 129 24 0:45 /file /tmp/dir2/file rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/file root@criu2:/tmp# btrfs --version btrfs-progs v4.4 root@criu2:/tmp# uname -a Linux criu2 4.4.0-8-generic #23-Ubuntu SMP Wed Feb 24 20:45:30 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
Quoted 3 lines
The issue here is that the "subvol=" mount option for the target of the bind mount is "/file" when no such subvolume actually exists. Is this intended? It's confusing to say the least, but seems like a bug to me.
Since btrfs mount subvol=<name> is a bind mount behind the scene, I'm not sure the mount info code distinguishes between bind mounts.
At the moment, this is something of a secret decoder ring where if you see subvolid=5 first, then anything after that other than / is just not true (can't be). Hence probably why both subvolid and subvol are listed for now; you kinda have to parse them both.
Signature · 1 line
Chris Murphy
Quoted 22 lines · Chris Murphy
Quoted 16 lines · Tycho Andersen
Hi all,
I'm seeing some strange behavior when bind mounting files from a btrfs subvolume. Consider the output below:
root@criu2:/tmp# mount -o loop /tmp/tester.btrfs /tmp/dir1 root@criu2:/tmp# touch dir1/file root@criu2:/tmp# sudo mount --bind dir1/file dir2/file root@criu2:/tmp# grep "/tmp/dir" /proc/self/mountinfo 128 24 0:45 / /tmp/dir1 rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/ 129 24 0:45 /file /tmp/dir2/file rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/file root@criu2:/tmp# btrfs --version btrfs-progs v4.4 root@criu2:/tmp# uname -a Linux criu2 4.4.0-8-generic #23-Ubuntu SMP Wed Feb 24 20:45:30 UTC 2016 x86_64 x86_64 x86_64 GNU/LinuxQuoted 3 lines
The issue here is that the "subvol=" mount option for the target of the bind mount is "/file" when no such subvolume actually exists. Is this intended? It's confusing to say the least, but seems like a bug to me.
Quoted 7 lines
Since btrfs mount subvol=<name> is a bind mount behind the scene, I'm not sure the mount info code distinguishes between bind mounts.
At the moment, this is something of a secret decoder ring where if you see subvolid=5 first, then anything after that other than / is just not true (can't be). Hence probably why both subvolid and subvol are listed for now; you kinda have to parse them both.
Ok, so if I'm trying to compare superblocks from userspace, I should look for subvolid and see if those match, and if that isn't present then look for subvol?
Tycho
Quoted 26 lines · Tycho Andersen
Quoted 25 lines · Chris Murphy
Quoted 16 lines · Tycho Andersen
Hi all,
I'm seeing some strange behavior when bind mounting files from a btrfs subvolume. Consider the output below:
root@criu2:/tmp# mount -o loop /tmp/tester.btrfs /tmp/dir1 root@criu2:/tmp# touch dir1/file root@criu2:/tmp# sudo mount --bind dir1/file dir2/file root@criu2:/tmp# grep "/tmp/dir" /proc/self/mountinfo 128 24 0:45 / /tmp/dir1 rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/ 129 24 0:45 /file /tmp/dir2/file rw,relatime shared:107 - btrfs /dev/loop0 rw,space_cache,subvolid=5,subvol=/file root@criu2:/tmp# btrfs --version btrfs-progs v4.4 root@criu2:/tmp# uname -a Linux criu2 4.4.0-8-generic #23-Ubuntu SMP Wed Feb 24 20:45:30 UTC 2016 x86_64 x86_64 x86_64 GNU/LinuxQuoted 3 lines
The issue here is that the "subvol=" mount option for the target of the bind mount is "/file" when no such subvolume actually exists. Is this intended? It's confusing to say the least, but seems like a bug to me.
Since btrfs mount subvol=<name> is a bind mount behind the scene, I'm not sure the mount info code distinguishes between bind mounts.
Quoted 8 lines
Quoted 4 lines
At the moment, this is something of a secret decoder ring where if you see subvolid=5 first, then anything after that other than / is just not true (can't be). Hence probably why both subvolid and subvol are listed for now; you kinda have to parse them both.
Ok, so if I'm trying to compare superblocks from userspace, I should look for subvolid and see if those match, and if that isn't present then look for subvol?
That depends on what you mean by 'compare superblocks'.
Internal to the kernel, there is exactly one super-block structure shared between all mounts of a given BTRFS filesystem, regardless of subvolume or bind-mount (the same is actually also true if you mount a XFS or ext4 filesystem more than once on the same system), so if you're trying to differentiate filesystems, you need to match on something like the block device or label.
If however, you're trying to differentiate where something is mounted from, you should be matching on both subvolid and subvol (I think, but I may be wrong), but that may not always work (I don't use bind mounts of BTRFS filesystems often outside of subvolume mounts, so I don't have much experience with this particular case).