[RFC PATCH 3/4] btrfs: undelete: introduce btrfs_undelete_subvolume

[Lu Fengqi <lufq.fnst@cn.fujitsu.com>]

The function will do the following things which are almost the opposite
of what btrfs_delete_subvolume() does:

1. link the subvolume to the parent specified;
2. clear root flag and set root_refs to 1;
3. add the subvol to the uuid_tree;
4. delete the orphan_item.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
---
 fs/btrfs/ioctl.c | 116 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 116 insertions(+)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index e0b5a8fb15e7..7a11c4f8e450 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1864,6 +1864,122 @@ static noinline int btrfs_ioctl_snap_create_v2(struct file *file,
 	return ret;
 }
 
+static int btrfs_undelete_subvolume(const struct path *parent,
+				    struct btrfs_root *root,
+				    const char *name, int namelen)
+{
+	struct inode *dir = d_inode(parent->dentry);
+	struct btrfs_fs_info *fs_info = btrfs_sb(dir->i_sb);
+	struct btrfs_root_item *root_item = &root->root_item;
+	struct btrfs_trans_handle *trans;
+	struct btrfs_block_rsv block_rsv;
+	struct dentry *dentry;
+	struct inode *inode;
+	u64 root_flags;
+	int ret;
+
+	btrfs_debug(fs_info, "Undelete subvolume %llu",
+				root->root_key.objectid);
+
+	/* only care about the intact subvolume */
+	if (btrfs_disk_key_objectid(&root_item->drop_progress) != 0)
+		return 0;
+
+	/* root_refs of destination parent root must not be 0 */
+	if (btrfs_root_refs(&BTRFS_I(dir)->root->root_item) == 0)
+		return -ENOENT;
+
+	ret = down_write_killable_nested(&dir->i_rwsem, I_MUTEX_PARENT);
+	if (ret == -EINTR)
+		return ret;
+
+	dentry = lookup_one_len(name, parent->dentry, namelen);
+	if (IS_ERR(dentry)) {
+		ret = PTR_ERR(dentry);
+		goto out_unlock;
+	}
+
+	down_write(&fs_info->subvol_sem);
+
+	ret = btrfs_may_create(dir, dentry);
+	if (ret)
+		goto out_up_write;
+
+	ret = btrfs_check_dir_item_collision(root, dir->i_ino, name, namelen);
+	if (ret)
+		goto out_up_write;
+
+	btrfs_init_block_rsv(&block_rsv, BTRFS_BLOCK_RSV_TEMP);
+	/*
+	 * 1 - parent dir inode
+	 * 2 - dir entries
+	 * 2 - root ref/backref
+	 * 1 - UUID item
+	 */
+	ret = btrfs_subvolume_reserve_metadata(root, &block_rsv, 6, false);
+	if (ret)
+		goto out_up_write;
+
+	trans = btrfs_start_transaction(BTRFS_I(dir)->root, 0);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		btrfs_subvolume_release_metadata(fs_info, &block_rsv);
+		goto out_up_write;
+	}
+
+	trans->block_rsv = &block_rsv;
+	trans->bytes_reserved = block_rsv.size;
+
+	ret = btrfs_link_subvol(trans, dir, root->root_key.objectid, name,
+				namelen);
+	if (ret)
+		goto fail;
+
+	/* clear BTRFS_ROOT_SUBVOL_DEAD root flag and set root_refs to 1*/
+	root_flags = btrfs_root_flags(root_item);
+	btrfs_set_root_flags(root_item,
+			     root_flags & ~BTRFS_ROOT_SUBVOL_DEAD);
+	btrfs_set_root_refs(root_item, 1);
+	ret = btrfs_update_root(trans, fs_info->tree_root,
+				&root->root_key, &root->root_item);
+	if (ret) {
+		btrfs_abort_transaction(trans, ret);
+		goto fail;
+	}
+
+	ret = btrfs_uuid_tree_add(trans, root_item->uuid, BTRFS_UUID_KEY_SUBVOL,
+				  root->root_key.objectid);
+	if (ret) {
+		btrfs_abort_transaction(trans, ret);
+		goto fail;
+	}
+
+	ret = btrfs_del_orphan_item(trans, fs_info->tree_root,
+				    root->root_key.objectid);
+	if (ret && ret != -ENOENT) {
+		btrfs_abort_transaction(trans, ret);
+		goto fail;
+	}
+fail:
+	trans->block_rsv = NULL;
+	trans->bytes_reserved = 0;
+	btrfs_subvolume_release_metadata(fs_info, &block_rsv);
+	ret = btrfs_commit_transaction(trans);
+	if (!ret) {
+		inode = btrfs_lookup_dentry(dir, dentry);
+		if (IS_ERR(inode))
+			return PTR_ERR(inode);
+		d_instantiate(dentry, inode);
+		fsnotify_mkdir(dir, dentry);
+	}
+out_up_write:
+	up_write(&fs_info->subvol_sem);
+	dput(dentry);
+out_unlock:
+	inode_unlock(dir);
+	return ret;
+}
+
 static noinline int btrfs_ioctl_subvol_getflags(struct file *file,
 						void __user *arg)
 {
-- 
2.18.0