Documentation/filesystems/automount-support.rst

Source file repositories/reference/linux-study-clean/Documentation/filesystems/automount-support.rst

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.. SPDX-License-Identifier: GPL-2.0

=================
Automount Support
=================


Support is available for filesystems that wish to do automounting
support (such as kAFS which can be found in fs/afs/ and NFS in
fs/nfs/). 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
======================

See section "Mount Traps" of  Documentation/filesystems/autofs.rst

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 separately.

To do expiration, you need to follow these steps:

 (1) Create at least one list off which the vfsmounts to be expired can be
     hung.

 (2) When a new mountpoint is created in the ->d_automount method, add
     the mnt to the list using mnt_set_expiry()::

             mnt_set_expiry(newmnt, &afs_vfsmounts);

 (3) 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

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