1. The PFS processes requests for UNMOUNT with the NORMAL, IMMEDIATE, and FORCE options as follows:
   • NORMAL. Synchronizes all data buffers to disk (if appropriate for this PFS). This saves all data changes to files in the file system that is being unmounted. If an I/O error occurs during this activity, the unmount request fails.
   • IMMEDIATE. Synchronizes all data buffers to disk (if appropriate for this PFS). If an I/O error occurs during this activity, the unmount request fails.
   • FORCE. Synchronizes all data buffers to disk (if appropriate for this PFS). If an I/O error occurs during this activity, the unmount proceeds anyway and data is lost.

   The difference between NORMAL and IMMEDIATE is whether the PFS is likely to find itself with any active inodes other than the one belonging to the root. The difference between IMMEDIATE and FORCE is whether the PFS continues if it encounters an I/O error while trying to synchronize data during the unmount.

2. The PFS frees any inodes that are still active, including the root inode, which is never explicitly inactivated.
3. The PFS reverses the vfs_mount and returns the file system to unready status.

Serialization: The whole file system is serialized under an exclusive latch at the time vfs_umount is called. No other vnode or VFS operations are running, although some may be in the PFS in a blocked state. See LFS-PFS control block serialization for more about serialization and blocking.