linux/Documentation/filesystems/automount-support.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3=================
   4Automount Support
   5=================
   6
   7
   8Support is available for filesystems that wish to do automounting
   9support (such as kAFS which can be found in fs/afs/ and NFS in
  10fs/nfs/). This facility includes allowing in-kernel mounts to be
  11performed and mountpoint degradation to be requested. The latter can
  12also be requested by userspace.
  13
  14
  15In-Kernel Automounting
  16======================
  17
  18See section "Mount Traps" of  Documentation/filesystems/autofs.rst
  19
  20Then from userspace, you can just do something like::
  21
  22        [root@andromeda root]# mount -t afs \#root.afs. /afs
  23        [root@andromeda root]# ls /afs
  24        asd  cambridge  cambridge.redhat.com  grand.central.org
  25        [root@andromeda root]# ls /afs/cambridge
  26        afsdoc
  27        [root@andromeda root]# ls /afs/cambridge/afsdoc/
  28        ChangeLog  html  LICENSE  pdf  RELNOTES-1.2.2
  29
  30And then if you look in the mountpoint catalogue, you'll see something like::
  31
  32        [root@andromeda root]# cat /proc/mounts
  33        ...
  34        #root.afs. /afs afs rw 0 0
  35        #root.cell. /afs/cambridge.redhat.com afs rw 0 0
  36        #afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0
  37
  38
  39Automatic Mountpoint Expiry
  40===========================
  41
  42Automatic expiration of mountpoints is easy, provided you've mounted the
  43mountpoint to be expired in the automounting procedure outlined separately.
  44
  45To do expiration, you need to follow these steps:
  46
  47 (1) Create at least one list off which the vfsmounts to be expired can be
  48     hung.
  49
  50 (2) When a new mountpoint is created in the ->d_automount method, add
  51     the mnt to the list using mnt_set_expiry()::
  52
  53             mnt_set_expiry(newmnt, &afs_vfsmounts);
  54
  55 (3) When you want mountpoints to be expired, call mark_mounts_for_expiry()
  56     with a pointer to this list. This will process the list, marking every
  57     vfsmount thereon for potential expiry on the next call.
  58
  59     If a vfsmount was already flagged for expiry, and if its usage count is 1
  60     (it's only referenced by its parent vfsmount), then it will be deleted
  61     from the namespace and thrown away (effectively unmounted).
  62
  63     It may prove simplest to simply call this at regular intervals, using
  64     some sort of timed event to drive it.
  65
  66The expiration flag is cleared by calls to mntput. This means that expiration
  67will only happen on the second expiration request after the last time the
  68mountpoint was accessed.
  69
  70If a mountpoint is moved, it gets removed from the expiration list. If a bind
  71mount is made on an expirable mount, the new vfsmount will not be on the
  72expiration list and will not expire.
  73
  74If a namespace is copied, all mountpoints contained therein will be copied,
  75and the copies of those that are on an expiration list will be added to the
  76same expiration list.
  77
  78
  79Userspace Driven Expiry
  80=======================
  81
  82As an alternative, it is possible for userspace to request expiry of any
  83mountpoint (though some will be rejected - the current process's idea of the
  84rootfs for example). It does this by passing the MNT_EXPIRE flag to
  85umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH.
  86
  87If the mountpoint in question is in referenced by something other than
  88umount() or its parent mountpoint, an EBUSY error will be returned and the
  89mountpoint will not be marked for expiration or unmounted.
  90
  91If the mountpoint was not already marked for expiry at that time, an EAGAIN
  92error will be given and it won't be unmounted.
  93
  94Otherwise if it was already marked and it wasn't referenced, unmounting will
  95take place as usual.
  96
  97Again, the expiration flag is cleared every time anything other than umount()
  98looks at a mountpoint.
  99