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+Support is available for filesystems that wish to do automounting support (such
+as kAFS which can be found in fs/afs/). This facility includes allowing
+in-kernel mounts to be performed and mountpoint degradation to be
+requested. The latter can also be requested by userspace.
+
+
+======================
+IN-KERNEL AUTOMOUNTING
+======================
+
+A filesystem can now mount another filesystem on one of its directories by the
+following procedure:
+
+ (1) Give the directory a follow_link() operation.
+
+     When the directory is accessed, the follow_link op will be called, and
+     it will be provided with the location of the mountpoint in the nameidata
+     structure (vfsmount and dentry).
+
+ (2) Have the follow_link() op do the following steps:
+
+     (a) Call do_kern_mount() to call the appropriate filesystem to set up a
+         superblock and gain a vfsmount structure representing it.
+
+     (b) Copy the nameidata provided as an argument and substitute the dentry
+         argument into it the copy.
+
+     (c) Call do_add_mount() to install the new vfsmount into the namespace's
+         mountpoint tree, thus making it accessible to userspace. Use the
+         nameidata set up in (b) as the destination.
+
+         If the mountpoint will be automatically expired, then do_add_mount()
+         should also be given the location of an expiration list (see further
+         down).
+
+     (d) Release the path in the nameidata argument and substitute in the new
+         vfsmount and its root dentry. The ref counts on these will need
+         incrementing.
+
+Then from userspace, you can just do something like:
+
+        [root@andromeda root]# mount -t afs \#root.afs. /afs
+        [root@andromeda root]# ls /afs
+        asd  cambridge  cambridge.redhat.com  grand.central.org
+        [root@andromeda root]# ls /afs/cambridge
+        afsdoc
+        [root@andromeda root]# ls /afs/cambridge/afsdoc/
+        ChangeLog  html  LICENSE  pdf  RELNOTES-1.2.2
+
+And then if you look in the mountpoint catalogue, you'll see something like:
+
+        [root@andromeda root]# cat /proc/mounts
+        ...
+        #root.afs. /afs afs rw 0 0
+        #root.cell. /afs/cambridge.redhat.com afs rw 0 0
+        #afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0
+
+
+===========================
+AUTOMATIC MOUNTPOINT EXPIRY
+===========================
+
+Automatic expiration of mountpoints is easy, provided you've mounted the
+mountpoint to be expired in the automounting procedure outlined above.
+
+To do expiration, you need to follow these steps:
+
+ (3) Create at least one list off which the vfsmounts to be expired can be
+     hung. Access to this list will be governed by the vfsmount_lock.
+
+ (4) In step (2c) above, the call to do_add_mount() should be provided with a
+     pointer to this list. It will hang the vfsmount off of it if it succeeds.
+
+ (5) When you want mountpoints to be expired, call mark_mounts_for_expiry()
+     with a pointer to this list. This will process the list, marking every
+     vfsmount thereon for potential expiry on the next call.
+
+     If a vfsmount was already flagged for expiry, and if its usage count is 1
+     (it's only referenced by its parent vfsmount), then it will be deleted
+     from the namespace and thrown away (effectively unmounted).
+
+     It may prove simplest to simply call this at regular intervals, using
+     some sort of timed event to drive it.
+
+The expiration flag is cleared by calls to mntput. This means that expiration
+will only happen on the second expiration request after the last time the
+mountpoint was accessed.
+
+If a mountpoint is moved, it gets removed from the expiration list. If a bind
+mount is made on an expirable mount, the new vfsmount will not be on the
+expiration list and will not expire.
+
+If a namespace is copied, all mountpoints contained therein will be copied,
+and the copies of those that are on an expiration list will be added to the
+same expiration list.
+
+
+=======================
+USERSPACE DRIVEN EXPIRY
+=======================
+
+As an alternative, it is possible for userspace to request expiry of any
+mountpoint (though some will be rejected - the current process's idea of the
+rootfs for example). It does this by passing the MNT_EXPIRE flag to
+umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH.
+
+If the mountpoint in question is in referenced by something other than
+umount() or its parent mountpoint, an EBUSY error will be returned and the
+mountpoint will not be marked for expiration or unmounted.
+
+If the mountpoint was not already marked for expiry at that time, an EAGAIN
+error will be given and it won't be unmounted.
+
+Otherwise if it was already marked and it wasn't referenced, unmounting will
+take place as usual.
+
+Again, the expiration flag is cleared every time anything other than umount()
+looks at a mountpoint.