swapon(8) tries to activate already active swap partition if the swap area is on DM device. Fixed by upstream patch:

commit b98ff23e879826d0b041a18ca1090428748b9ef3
Author: Alexey Gladkov
Date:   Wed Dec 15 02:14:18 2010 +0300

    swapon: Canonicalize swap device
    
    Swapon checks whether a swap device is active by searching for the
    device name in /proc/swaps. /proc/swaps always specifies the path
    to real device file, even if the path to real device file, even
    if symlink was passed to the swapon() system call.
    
    This differs from /proc/mounts semantics where each string contains
    exactly the same device name as it was passed to the mount*() system call.
    
    If a swap partition resides on lvm, libblkid returns a name in
    form /dev/mapper/*, but now there are symlinks pointing to device
    files /dev/dm-*, resulting to /proc/swaps containing /dev/dm-*,
    but swapon still looks for /dev/mapper/* and tries to activate
    the swap partition again.
    
    [kzak: - remove unnecessary changes from
                        is_in_proc_swaps()]
    
    Signed-off-by: Alexey Gladkov
    Tested-by: Petr Uzel
    Signed-off-by: Karel