LFS processing

NORMAL. The LFS checks to make sure no user is using any of the files in the file system that is to be unmounted, and passes the request to the PFS via vfs_umount. If files in this file system are being accessed, the LFS rejects the unmount request.
DRAIN. The LFS checks to make sure that no user is accessing any of the files in the file system that is to be unmounted, and passes the request to the PFS via vfs_umount. If files in this file system are being accessed, the LFS waits until all activity has ceased, and then passes the request to the PFS.
RESET. The LFS cancels a previous unmount drain request. The file system goes back to the normal mounted state.
IMMEDIATE. The LFS stops further user access to the file system that is being unmounted. Any attempt to access files in this file system receives an error return code. The LFS then passes the request to the PFS via vfs_umount.
UNMOUNT with IMMEDIATE can be used to override a previous UNMOUNT DRAIN request for a file system.
FORCE. The LFS stops further user access to the file system that is being unmounted. Any attempt to access files in this file system receives an error return code. The LFS passes the request to the PFS via vfs_umount.
UNMOUNT with FORCE can be used to unmount a file system even if I/O errors are being received from the underlying device.

An IMMEDIATE unmount request must be issued before a FORCE unmount can be requested.
REMOUNT. The LFS handles this like an IMMEDIATE unmount followed by a mount. User access is suspended while the operations are in progress. vfs_vget is used to establish the vnode/inode bindings so that the remount is not disruptive to the users.