PowerPath: powermt remove reports "Device is busy" on Linux

On Linux, while the back-end IO paths are still alive (i.e, HBAs could still see the target Lun), cannot remove Powerpath pseudo-device, as powermt remove reports "Device is Busy"; Verified that the target pseudo devices has already been removed from the management of LVM or Oracle ASM, etcs Rebooting the host does not work, either.

Some daemon is still keeping the pseudo device and/or its native paths OPEN. This is how Powerpath determines if a device is busy, according to PP Dev team:

PowerPath just keeps track of opens sent down from the userland to determine if any process (or processes) is using a device and if any opens are active, then PowerPath will not allow to remove the device from its control.  This includes the number of opens on a device before PowerPath is started.  But it does not keep track of the processes (like the process name or process id) that open the device(s).    Hence it is not possible for PowerPath to list which process may have opened a device.

powermt unmanage dev=[target pseudo device] , followed by a host reboot, might be able to remove the target device from PP management as desired.