Btrfs: change how subvolumes are organized

btrfs allows subvolumes and snapshots anywhere in the directory tree.
If we snapshot a subvolume that contains a link to other subvolume
called subvolA, subvolA can be accessed through both the original
subvolume and the snapshot. This is similar to creating hard link to
directory, and has the very similar problems.

The aim of this patch is enforcing there is only one access point to
each subvolume. Only the first directory entry (the one added when
the subvolume/snapshot was created) is treated as valid access point.
The first directory entry is distinguished by checking root forward
reference. If the corresponding root forward reference is missing,
we know the entry is not the first one.

This patch also adds snapshot/subvolume rename support, the code
allows rename subvolume link across subvolumes.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -41,6 +41,7 @@
 
 static struct extent_io_ops btree_extent_io_ops;
 static void end_workqueue_fn(struct btrfs_work *work);
+static void free_fs_root(struct btrfs_root *root);
 
 static atomic_t btrfs_bdi_num = ATOMIC_INIT(0);
 
@@ -951,14 +952,16 @@ static int find_and_setup_root(struct btrfs_root *tree_root,
 		     root, fs_info, objectid);
 	ret = btrfs_find_last_root(tree_root, objectid,
 				   &root->root_item, &root->root_key);
+	if (ret > 0)
+		return -ENOENT;
 	BUG_ON(ret);
 
 	generation = btrfs_root_generation(&root->root_item);
 	blocksize = btrfs_level_size(root, btrfs_root_level(&root->root_item));
 	root->node = read_tree_block(root, btrfs_root_bytenr(&root->root_item),
 				     blocksize, generation);
-	root->commit_root = btrfs_root_node(root);
 	BUG_ON(!root->node);
+	root->commit_root = btrfs_root_node(root);
 	return 0;
 }
 
@@ -1176,39 +1179,66 @@ struct btrfs_root *btrfs_read_fs_root_no_name(struct btrfs_fs_info *fs_info,
 		return fs_info->dev_root;
 	if (location->objectid == BTRFS_CSUM_TREE_OBJECTID)
 		return fs_info->csum_root;
-
+again:
+	spin_lock(&fs_info->fs_roots_radix_lock);
 	root = radix_tree_lookup(&fs_info->fs_roots_radix,
 				 (unsigned long)location->objectid);
+	spin_unlock(&fs_info->fs_roots_radix_lock);
 	if (root)
 		return root;
 
+	ret = btrfs_find_orphan_item(fs_info->tree_root, location->objectid);
+	if (ret == 0)
+		ret = -ENOENT;
+	if (ret < 0)
+		return ERR_PTR(ret);
+
 	root = btrfs_read_fs_root_no_radix(fs_info->tree_root, location);
 	if (IS_ERR(root))
 		return root;
 
+	WARN_ON(btrfs_root_refs(&root->root_item) == 0);
 	set_anon_super(&root->anon_super, NULL);
 
+	ret = radix_tree_preload(GFP_NOFS & ~__GFP_HIGHMEM);
+	if (ret)
+		goto fail;
+
+	spin_lock(&fs_info->fs_roots_radix_lock);
 	ret = radix_tree_insert(&fs_info->fs_roots_radix,
 				(unsigned long)root->root_key.objectid,
 				root);
+	if (ret == 0)
+		root->in_radix = 1;
+	spin_unlock(&fs_info->fs_roots_radix_lock);
+	radix_tree_preload_end();
 	if (ret) {
-		free_extent_buffer(root->node);
-		kfree(root);
-		return ERR_PTR(ret);
+		if (ret == -EEXIST) {
+			free_fs_root(root);
+			goto again;
+		}
+		goto fail;
 	}
-	if (!(fs_info->sb->s_flags & MS_RDONLY)) {
-		ret = btrfs_find_dead_roots(fs_info->tree_root,
-					    root->root_key.objectid);
-		BUG_ON(ret);
+
+	ret = btrfs_find_dead_roots(fs_info->tree_root,
+				    root->root_key.objectid);
+	WARN_ON(ret);
+
+	if (!(fs_info->sb->s_flags & MS_RDONLY))
 		btrfs_orphan_cleanup(root);
-	}
+
 	return root;
+fail:
+	free_fs_root(root);
+	return ERR_PTR(ret);
 }
 
 struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 				      struct btrfs_key *location,
 				      const char *name, int namelen)
 {
+	return btrfs_read_fs_root_no_name(fs_info, location);
+#if 0
 	struct btrfs_root *root;
 	int ret;
 
@@ -1225,7 +1255,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 		kfree(root);
 		return ERR_PTR(ret);
 	}
-#if 0
+
 	ret = btrfs_sysfs_add_root(root);
 	if (ret) {
 		free_extent_buffer(root->node);
@@ -1233,9 +1263,9 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 		kfree(root);
 		return ERR_PTR(ret);
 	}
-#endif
 	root->in_sysfs = 1;
 	return root;
+#endif
 }
 
 static int btrfs_congested_fn(void *congested_data, int bdi_bits)
@@ -2229,20 +2259,25 @@ int write_ctree_super(struct btrfs_trans_handle *trans,
 
 int btrfs_free_fs_root(struct btrfs_fs_info *fs_info, struct btrfs_root *root)
 {
-	WARN_ON(!RB_EMPTY_ROOT(&root->inode_tree));
+	spin_lock(&fs_info->fs_roots_radix_lock);
 	radix_tree_delete(&fs_info->fs_roots_radix,
 			  (unsigned long)root->root_key.objectid);
+	spin_unlock(&fs_info->fs_roots_radix_lock);
+	free_fs_root(root);
+	return 0;
+}
+
+static void free_fs_root(struct btrfs_root *root)
+{
+	WARN_ON(!RB_EMPTY_ROOT(&root->inode_tree));
 	if (root->anon_super.s_dev) {
 		down_write(&root->anon_super.s_umount);
 		kill_anon_super(&root->anon_super);
 	}
-	if (root->node)
-		free_extent_buffer(root->node);
-	if (root->commit_root)
-		free_extent_buffer(root->commit_root);
+	free_extent_buffer(root->node);
+	free_extent_buffer(root->commit_root);
 	kfree(root->name);
 	kfree(root);
-	return 0;
 }
 
 static int del_fs_roots(struct btrfs_fs_info *fs_info)