99 lines
3.5 KiB
ReStructuredText
99 lines
3.5 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=================
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Automount Support
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=================
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Support is available for filesystems that wish to do automounting
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support (such as kAFS which can be found in fs/afs/ and NFS in
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fs/nfs/). This facility includes allowing in-kernel mounts to be
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performed and mountpoint degradation to be requested. The latter can
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also be requested by userspace.
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In-Kernel Automounting
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======================
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See section "Mount Traps" of Documentation/filesystems/autofs.rst
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Then from userspace, you can just do something like::
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[root@andromeda root]# mount -t afs \#root.afs. /afs
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[root@andromeda root]# ls /afs
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asd cambridge cambridge.redhat.com grand.central.org
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[root@andromeda root]# ls /afs/cambridge
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afsdoc
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[root@andromeda root]# ls /afs/cambridge/afsdoc/
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ChangeLog html LICENSE pdf RELNOTES-1.2.2
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And then if you look in the mountpoint catalogue, you'll see something like::
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[root@andromeda root]# cat /proc/mounts
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...
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#root.afs. /afs afs rw 0 0
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#root.cell. /afs/cambridge.redhat.com afs rw 0 0
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#afsdoc. /afs/cambridge.redhat.com/afsdoc afs rw 0 0
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Automatic Mountpoint Expiry
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===========================
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Automatic expiration of mountpoints is easy, provided you've mounted the
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mountpoint to be expired in the automounting procedure outlined separately.
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To do expiration, you need to follow these steps:
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(1) Create at least one list off which the vfsmounts to be expired can be
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hung.
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(2) When a new mountpoint is created in the ->d_automount method, add
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the mnt to the list using mnt_set_expiry()::
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mnt_set_expiry(newmnt, &afs_vfsmounts);
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(3) When you want mountpoints to be expired, call mark_mounts_for_expiry()
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with a pointer to this list. This will process the list, marking every
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vfsmount thereon for potential expiry on the next call.
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If a vfsmount was already flagged for expiry, and if its usage count is 1
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(it's only referenced by its parent vfsmount), then it will be deleted
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from the namespace and thrown away (effectively unmounted).
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It may prove simplest to simply call this at regular intervals, using
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some sort of timed event to drive it.
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The expiration flag is cleared by calls to mntput. This means that expiration
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will only happen on the second expiration request after the last time the
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mountpoint was accessed.
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If a mountpoint is moved, it gets removed from the expiration list. If a bind
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mount is made on an expirable mount, the new vfsmount will not be on the
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expiration list and will not expire.
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If a namespace is copied, all mountpoints contained therein will be copied,
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and the copies of those that are on an expiration list will be added to the
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same expiration list.
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Userspace Driven Expiry
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=======================
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As an alternative, it is possible for userspace to request expiry of any
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mountpoint (though some will be rejected - the current process's idea of the
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rootfs for example). It does this by passing the MNT_EXPIRE flag to
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umount(). This flag is considered incompatible with MNT_FORCE and MNT_DETACH.
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If the mountpoint in question is in referenced by something other than
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umount() or its parent mountpoint, an EBUSY error will be returned and the
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mountpoint will not be marked for expiration or unmounted.
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If the mountpoint was not already marked for expiry at that time, an EAGAIN
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error will be given and it won't be unmounted.
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Otherwise if it was already marked and it wasn't referenced, unmounting will
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take place as usual.
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Again, the expiration flag is cleared every time anything other than umount()
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looks at a mountpoint.
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