Unexport do_add_mount() and add in follow_automount(), not ->d_automount()

Unexport do_add_mount() and make ->d_automount() return the vfsmount to be
added rather than calling do_add_mount() itself.  follow_automount() will then
do the addition.

This slightly complicates things as ->d_automount() normally wants to add the
new vfsmount to an expiration list and start an expiration timer.  The problem
with that is that the vfsmount will be deleted if it has a refcount of 1 and
the timer will not repeat if the expiration list is empty.

To this end, we require the vfsmount to be returned from d_automount() with a
refcount of (at least) 2.  One of these refs will be dropped unconditionally.
In addition, follow_automount() must get a 3rd ref around the call to
do_add_mount() lest it eat a ref and return an error, leaving the mount we
have open to being expired as we would otherwise have only 1 ref on it.

d_automount() should also add the the vfsmount to the expiration list (by
calling mnt_set_expiry()) and start the expiration timer before returning, if
this mechanism is to be used.  The vfsmount will be unlinked from the
expiration list by follow_automount() if do_add_mount() fails.

This patch also fixes the call to do_add_mount() for AFS to propagate the mount
flags from the parent vfsmount.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 14 insertions, 9 deletions

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 3c4b2f1b64d0..94cf97b901d7 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -933,15 +933,20 @@ struct dentry_operations {
         dynamic_dname() helper function is provided to take care of this.
 
   d_automount: called when an automount dentry is to be traversed (optional).
-        This should create a new VFS mount record, mount it on the directory
-        and return the record to the caller.  The caller is supplied with a
-        path parameter giving the automount directory to describe the automount
-        target and the parent VFS mount record to provide inheritable mount
-        parameters.  NULL should be returned if someone else managed to make
-        the automount first.  If the automount failed, then an error code
-        should be returned.  If -EISDIR is returned, then the directory will
-        be treated as an ordinary directory and returned to pathwalk to
-        continue walking.
+        This should create a new VFS mount record and return the record to the
+        caller.  The caller is supplied with a path parameter giving the
+        automount directory to describe the automount target and the parent
+        VFS mount record to provide inheritable mount parameters.  NULL should
+        be returned if someone else managed to make the automount first.  If
+        the vfsmount creation failed, then an error code should be returned.
+        If -EISDIR is returned, then the directory will be treated as an
+        ordinary directory and returned to pathwalk to continue walking.
+
+        If a vfsmount is returned, the caller will attempt to mount it on the
+        mountpoint and will remove the vfsmount from its expiration list in
+        the case of failure.  The vfsmount should be returned with 2 refs on
+        it to prevent automatic expiration - the caller will clean up the
+        additional ref.
 
         This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
         dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the