Can not move tmpfs mounts

Can not move tmpfs mounts

[Phillip Susi]

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mount --move fails with -EINVAL on tmpfs mounts.  Is there some reason
for this, or is it a long standing kernel bug?

A bind mount followed by unmounting the original works, but seems like
this workaround should not be needed.
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Re: Can not move tmpfs mounts

[Karel Zak]

On Tue, Nov 24, 2015 at 10:05:42PM -0500, Phillip Susi wrote:
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> mount --move fails with -EINVAL on tmpfs mounts.  Is there some reason
> for this, or is it a long standing kernel bug?

Are you sure the problem is tmpfs? I guess you have problem with
propagation flags (shared vs. private), probably systemd based system
where all is shared by default.

# mount -t tmpfs tmpfs  /mnt/test

# strace -e mount mount --move /mnt/test /mnt/test2
mount("/mnt/test", "/mnt/test2", 0x563e27385f00, MS_MGC_VAL|MS_MOVE, NULL) = -1 EINVAL (Invalid argument)
mount: bad option. Note that moving a mount residing under a shared
       mount is unsupported.

See the mount(8) output message (it's also in mount(8) man page),
let's fix it:

# findmnt -o TARGET,PROPAGATION /mnt
TARGET PROPAGATION
/mnt   shared

# mount --make-private /mnt

# strace -e mount mount --move /mnt/test /mnt/test2 
mount("/mnt/test", "/mnt/test2", 0x55a092f54f00, MS_MGC_VAL|MS_MOVE, NULL) = 0


Note that the problem is not with the mountpoint (/mnt/test in my
example), but with parent -- unfortunately usually "/" ("/mnt" in my
example).

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: Can not move tmpfs mounts

[Phillip Susi]

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On 11/25/2015 04:39 AM, Karel Zak wrote:
> Are you sure the problem is tmpfs? I guess you have problem with 
> propagation flags (shared vs. private), probably systemd based
> system where all is shared by default.

I could have sworn it worked with an ext4 fs, but yea, you are
right... it is the shared flag.  Why can't you move a mount point when
it is in a shared namespace?


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