linux-kernel - confusing mountinfo output when bind-mounting files

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Message-ID: <20160321152144.GL8544@smitten>
Date:	Mon, 21 Mar 2016 09:21:44 -0600
From:	Tycho Andersen <tycho.andersen@...onical.com>
To:	linux-btrfs@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: 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@...u2:/tmp# mount -o loop /tmp/tester.btrfs /tmp/dir1
root@...u2:/tmp# touch dir1/file
root@...u2:/tmp# sudo mount --bind dir1/file dir2/file
root@...u2:/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@...u2:/tmp# btrfs --version
btrfs-progs v4.4
root@...u2:/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