Re: loop subsystem corrupted after mounting multiple btrfs sub-volumes

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Feb 26, 2016 at 11:39:11AM -0500, Austin S. Hemmelgarn wrote:

> That's just it though, from what I can tell based on what I've seen
> and what you said above, mount(8) isn't doing things correctly in
> this case.  If we were to do this with something like XFS or ext4,
> the filesystem would probably end up completely messed up just
> because of the log replay code (assuming they actually mount the
> second time, I'm not sure what XFS would do in this case, but I
> believe that ext4 would allow the mount as long as the mmp feature
> is off).  It would make sense that this behavior wouldn't have been
> noticed before (and probably wouldn't have mattered even if it had
> been), because most filesystems don't allow multiple mounts even if
> they're all RO, and most people don't try to mount other filesystems
> multiple times as a result of this.

They most certainly do.  The problem is mount(8) treatment of -o loop -
you can mount e.g. ext4 many times, it'll just get you extra references
to the same struct super_block from those new vfsmounts.  IOW, that'll
behave the same way as if you were doing mount --bind on subsequent ones.

And as far as kernel is concerned, /dev/loop* isn't special in any respects;
if you do explicit losetup and mount the resulting /dev/loop<n> as many
times as you wish, it'll work just fine.  And from the kernel POV it's not
different from what it sees with -o loop; setting the loop device up is
done first by separate syscall, then mount(2) for that device is issued.

It's mount(8) that screws up here.