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Documentation: remove outdated information from automount-support.txt

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The guidelines for adding automount support to a filesystem
in filesystems/automount-support.txt is out or date.
filesystems/autofs4.txt contains more current text, so replace
the out-of-date content with a reference to that.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
NeilBrown 2015-03-23 13:37:38 +11:00 committed by Al Viro
parent 89076bc319
commit 99ff6cf0e6
1 changed files with 13 additions and 38 deletions
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51
Documentation/filesystems/automount-support.txt
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@ -1,41 +1,15 @@
Support is available for filesystems that wish to do automounting support (such
as kAFS which can be found in fs/afs/). This facility includes allowing
in-kernel mounts to be performed and mountpoint degradation to be
requested. The latter can also be requested by userspace.
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
======================
A filesystem can now mount another filesystem on one of its directories by the
following procedure:
(1) Give the directory a follow_link() operation.
When the directory is accessed, the follow_link op will be called, and
it will be provided with the location of the mountpoint in the nameidata
structure (vfsmount and dentry).
(2) Have the follow_link() op do the following steps:
(a) Call vfs_kern_mount() to call the appropriate filesystem to set up a
superblock and gain a vfsmount structure representing it.
(b) Copy the nameidata provided as an argument and substitute the dentry
argument into it the copy.
(c) Call do_add_mount() to install the new vfsmount into the namespace's
mountpoint tree, thus making it accessible to userspace. Use the
nameidata set up in (b) as the destination.
If the mountpoint will be automatically expired, then do_add_mount()
should also be given the location of an expiration list (see further
down).
(d) Release the path in the nameidata argument and substitute in the new
vfsmount and its root dentry. The ref counts on these will need
incrementing.
See section "Mount Traps" of Documentation/filesystems/autofs4.txt
Then from userspace, you can just do something like:
@ -61,17 +35,18 @@ AUTOMATIC MOUNTPOINT EXPIRY
===========================
Automatic expiration of mountpoints is easy, provided you've mounted the
mountpoint to be expired in the automounting procedure outlined above.
mountpoint to be expired in the automounting procedure outlined separately.
To do expiration, you need to follow these steps:
(3) Create at least one list off which the vfsmounts to be expired can be
hung. Access to this list will be governed by the vfsmount_lock.
(1) Create at least one list off which the vfsmounts to be expired can be
hung.
(4) In step (2c) above, the call to do_add_mount() should be provided with a
pointer to this list. It will hang the vfsmount off of it if it succeeds.
(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);
(5) When you want mountpoints to be expired, call mark_mounts_for_expiry()
(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.