Can not move tmpfs mounts

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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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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  <[email protected]>
 http://karelzak.blogspot.com

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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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