Messages in this thread | | | Date | Mon, 21 Mar 2016 11:29:36 -0600 | From | Tycho Andersen <> | Subject | Re: confusing mountinfo output when bind-mounting files |
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On Mon, Mar 21, 2016 at 11:22:06AM -0600, Chris Murphy wrote: > On Mon, Mar 21, 2016 at 9:21 AM, Tycho Andersen > <tycho.andersen@canonical.com> wrote: > > 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. > > 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
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