Re: Two identical copies of an image mounted result in changes to both images if only one is modified

[Gabriel de Perthuis <g2p.code@gmail.com>]

On Thu, 20 Jun 2013 10:16:22 +0100, Hugo Mills wrote:
> On Thu, Jun 20, 2013 at 10:47:53AM +0200, Clemens Eisserer wrote:
>> Hi,
>> 
>> I've observed a rather strange behaviour while trying to mount two
>> identical copies of the same image to different mount points.
>> Each modification to one image is also performed in the second one.

>> touch m2/hello
>> ls -la m1  //will now also include a file calles "hello"
>> 
>> Is this behaviour intentional and known or should I create a bug-report?
> 
>    It's known, and not desired behaviour. The problem is that you've
> ended up with two filesystems with the same UUID, and the FS code gets
> rather confused about that. The same problem exists with LVM snapshots
> (or other block-device-layer copies).
> 
>    The solution is a combination of a tool to scan an image and change
> the UUID (offline), and of some code in the kernel that detects when
> it's being told about a duplicate image (rather than an additional
> device in the same FS). Neither of these has been written yet, I'm
> afraid.

To clarify, the loop devices are properly distinct, but the first
device ends up mounted twice.

I've had a look at the vfs code, and it doesn't seem to be uuid-aware,
which makes sense because the uuid is a property of the superblock and
the fs structure doesn't expose it.  It's a Btrfs problem.

Instead of redirecting to a different block device, Btrfs could and
should refuse to mount an already-mounted superblock when the block
device doesn't match, somewhere in or below btrfs_mount.  Registering
extra, distinct superblocks for an already mounted raid is a different
matter, but that isn't done through the mount syscall anyway.

>> I've deleted quite a bunch of files on my production system because of this...
> 
>    Oops. I'm sorry to hear that. :(
> 
>    Hugo.