linux/Documentation/filesystems/automount-support.txt
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   1Support is available for filesystems that wish to do automounting support (such
   2as kAFS which can be found in fs/afs/). This facility includes allowing
   3in-kernel mounts to be performed and mountpoint degradation to be
   4requested. The latter can also be requested by userspace.
   5
   6
   7======================
   8IN-KERNEL AUTOMOUNTING
   9======================
  10
  11A filesystem can now mount another filesystem on one of its directories by the
  12following procedure:
  13
  14 (1) Give the directory a follow_link() operation.
  15
  16     When the directory is accessed, the follow_link op will be called, and
  17     it will be provided with the location of the mountpoint in the nameidata
  18     structure (vfsmount and dentry).
  19
  20 (2) Have the follow_link() op do the following steps:
  21
  22     (a) Call vfs_kern_mount() to call the appropriate filesystem to set up a
  23         superblock and gain a vfsmount structure representing it.
  24
  25     (b) Copy the nameidata provided as an argument and substitute the dentry
  26         argument into it the copy.
  27
  28     (c) Call do_add_mount() to install the new vfsmount into the namespace's
  29         mountpoint tree, thus making it accessible to userspace. Use the
  30         nameidata set up in (b) as the destination.
  31
  32         If the mountpoint will be automatically expired, then do_add_mount()
  33         should also be given the location of an expiration list (see further
  34         down).
  35
  36     (d) Release the path in the nameidata argument and substitute in the new
  37         vfsmount and its root dentry. The ref counts on these will need
  38         incrementing.
  39
  40Then from userspace, you can just do something like:
  41
  42        [root@andromeda root]# mount -t afs \#root.afs. /afs
  43        [root@andromeda root]# ls /afs
  44        asd  cambridge  cambridge.redhat.com  grand.central.org
  45        [root@andromeda root]# ls /afs/cambridge
  46        afsdoc
  47        [root@andromeda root]# ls /afs/cambridge/afsdoc/
  48        ChangeLog  html  LICENSE  pdf  RELNOTES-1.2.2
  49
  50And then if you look in the mountpoint catalogue, you'll see something like:
  51
  52        [root@andromeda root]# cat /proc/mounts
  53        ...
  54        #root.afs. /afs afs rw 0 0
  55        #root.cell. /afs/cambridge.redhat.com afs rw 0 0
  56        #afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0
  57
  58
  59===========================
  60AUTOMATIC MOUNTPOINT EXPIRY
  61===========================
  62
  63Automatic expiration of mountpoints is easy, provided you've mounted the
  64mountpoint to be expired in the automounting procedure outlined above.
  65
  66To do expiration, you need to follow these steps:
  67
  68 (3) Create at least one list off which the vfsmounts to be expired can be
  69     hung. Access to this list will be governed by the vfsmount_lock.
  70
  71 (4) In step (2c) above, the call to do_add_mount() should be provided with a
  72     pointer to this list. It will hang the vfsmount off of it if it succeeds.
  73
  74 (5) When you want mountpoints to be expired, call mark_mounts_for_expiry()
  75     with a pointer to this list. This will process the list, marking every
  76     vfsmount thereon for potential expiry on the next call.
  77
  78     If a vfsmount was already flagged for expiry, and if its usage count is 1
  79     (it's only referenced by its parent vfsmount), then it will be deleted
  80     from the namespace and thrown away (effectively unmounted).
  81
  82     It may prove simplest to simply call this at regular intervals, using
  83     some sort of timed event to drive it.
  84
  85The expiration flag is cleared by calls to mntput. This means that expiration
  86will only happen on the second expiration request after the last time the
  87mountpoint was accessed.
  88
  89If a mountpoint is moved, it gets removed from the expiration list. If a bind
  90mount is made on an expirable mount, the new vfsmount will not be on the
  91expiration list and will not expire.
  92
  93If a namespace is copied, all mountpoints contained therein will be copied,
  94and the copies of those that are on an expiration list will be added to the
  95same expiration list.
  96
  97
  98=======================
  99USERSPACE DRIVEN EXPIRY
 100=======================
 101
 102As an alternative, it is possible for userspace to request expiry of any
 103mountpoint (though some will be rejected - the current process's idea of the
 104rootfs for example). It does this by passing the MNT_EXPIRE flag to
 105umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH.
 106
 107If the mountpoint in question is in referenced by something other than
 108umount() or its parent mountpoint, an EBUSY error will be returned and the
 109mountpoint will not be marked for expiration or unmounted.
 110
 111If the mountpoint was not already marked for expiry at that time, an EAGAIN
 112error will be given and it won't be unmounted.
 113
 114Otherwise if it was already marked and it wasn't referenced, unmounting will
 115take place as usual.
 116
 117Again, the expiration flag is cleared every time anything other than umount()
 118looks at a mountpoint.
 119
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