path: root/Documentation/filesystems/automount-support.txt
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authorLinus Torvalds <>2005-04-16 15:20:36 -0700
committerLinus Torvalds <>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/filesystems/automount-support.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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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.
+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
+ [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/ afs rw 0 0
+ #afsdoc. /afs/ afs rw 0 0
+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.
+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.