afs: Change dynroot to create contents on demand

Change the AFS dynamic root to do things differently:

 (1) Rather than having the creation of cell records create inodes and
     dentries for cell mountpoints, create them on demand during lookup.

     This simplifies cell management and locking as we no longer have to
     create these objects in advance *and* on speculative lookup by the
     user for a cell that isn't precreated.

 (2) Rather than using the libfs dentry-based readdir (the dentries now no
     longer exist until accessed from (1)), have readdir generate the
     contents by reading the list of cells.  The @cell symlinks get pushed
     in positions 2 and 3 if rootcell has been configured.

 (3) Make the @cell symlink dentries persist for the life of the superblock
     or until reclaimed, but make cell mountpoints disappear immediately if
     unused.

     It's not perfect as someone doing an "ls -l /afs" may create a whole
     bunch of dentries which will be garbage collected immediately.  But
     any dentry that gets automounted will be pinned by the mount, so it
     shouldn't be too bad.

 (4) Allocate the inode numbers for the cell mountpoints from an IDR to
     prevent duplicates appearing in the event it cycles round.  The number
     allocated from the IDR is doubled to provide two inode numbers - one
     for the normal cell name (RO) and one for the dotted cell name (RW).

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20250224234154.2014840-8-dhowells@redhat.com/ # v1
Link: https://lore.kernel.org/r/20250310094206.801057-4-dhowells@redhat.com/ # v4

Author: dhowells

fs/afs/cell.c

@@ -204,7 +204,13 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 	cell->dns_status = vllist->status;
 	smp_store_release(&cell->dns_lookup_count, 1); /* vs source/status */
 	atomic_inc(&net->cells_outstanding);
+	ret = idr_alloc_cyclic(&net->cells_dyn_ino, cell,
+			       2, INT_MAX / 2, GFP_KERNEL);
+	if (ret < 0)
+		goto error;
+	cell->dynroot_ino = ret;
 	cell->debug_id = atomic_inc_return(&cell_debug_id);
+
 	trace_afs_cell(cell->debug_id, 1, 0, afs_cell_trace_alloc);
 
 	_leave(" = %p", cell);
@@ -513,6 +519,7 @@ static void afs_cell_destroy(struct rcu_head *rcu)
 	afs_put_vlserverlist(net, rcu_access_pointer(cell->vl_servers));
 	afs_unuse_cell(net, cell->alias_of, afs_cell_trace_unuse_alias);
 	key_put(cell->anonymous_key);
+	idr_remove(&net->cells_dyn_ino, cell->dynroot_ino);
 	kfree(cell->name - 1);
 	kfree(cell);
 
@@ -706,7 +713,6 @@ static int afs_activate_cell(struct afs_net *net, struct afs_cell *cell)
 	if (cell->proc_link.next)
 		cell->proc_link.next->pprev = &cell->proc_link.next;
 
-	afs_dynroot_mkdir(net, cell);
 	mutex_unlock(&net->proc_cells_lock);
 	return 0;
 }
@@ -723,7 +729,6 @@ static void afs_deactivate_cell(struct afs_net *net, struct afs_cell *cell)
 	mutex_lock(&net->proc_cells_lock);
 	if (!hlist_unhashed(&cell->proc_link))
 		hlist_del_rcu(&cell->proc_link);
-	afs_dynroot_rmdir(net, cell);
 	mutex_unlock(&net->proc_cells_lock);
 
 	_leave("");