119 lines
4.6 KiB
Plaintext
119 lines
4.6 KiB
Plaintext
Support is available for filesystems that wish to do automounting support (such
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as kAFS which can be found in fs/afs/). This facility includes allowing
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in-kernel mounts to be performed and mountpoint degradation to be
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requested. The latter can also be requested by userspace.
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======================
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IN-KERNEL AUTOMOUNTING
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======================
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A filesystem can now mount another filesystem on one of its directories by the
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following procedure:
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(1) Give the directory a follow_link() operation.
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When the directory is accessed, the follow_link op will be called, and
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it will be provided with the location of the mountpoint in the nameidata
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structure (vfsmount and dentry).
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(2) Have the follow_link() op do the following steps:
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(a) Call vfs_kern_mount() to call the appropriate filesystem to set up a
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superblock and gain a vfsmount structure representing it.
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(b) Copy the nameidata provided as an argument and substitute the dentry
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argument into it the copy.
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(c) Call do_add_mount() to install the new vfsmount into the namespace's
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mountpoint tree, thus making it accessible to userspace. Use the
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nameidata set up in (b) as the destination.
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If the mountpoint will be automatically expired, then do_add_mount()
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should also be given the location of an expiration list (see further
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down).
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(d) Release the path in the nameidata argument and substitute in the new
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vfsmount and its root dentry. The ref counts on these will need
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incrementing.
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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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===========================
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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 above.
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To do expiration, you need to follow these steps:
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(3) Create at least one list off which the vfsmounts to be expired can be
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hung. Access to this list will be governed by the vfsmount_lock.
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(4) In step (2c) above, the call to do_add_mount() should be provided with a
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pointer to this list. It will hang the vfsmount off of it if it succeeds.
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(5) 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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=======================
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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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