start_transaction(struct btrfs_root *root, unsigned int num_items,
		  unsigned int type, enum btrfs_reserve_flush_enum flush,
		  bool enforce_qgroups)
{
	struct btrfs_fs_info *fs_info = root->fs_info;
	struct btrfs_block_rsv *delayed_refs_rsv = &fs_info->delayed_refs_rsv;
	struct btrfs_trans_handle *h;
	struct btrfs_transaction *cur_trans;
	u64 num_bytes = 0;
	u64 qgroup_reserved = 0;
	bool reloc_reserved = false;
	bool do_chunk_alloc = false;
	int ret;

	if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state))
		return ERR_PTR(-EROFS);

	if (current->journal_info) {
		WARN_ON(type & TRANS_EXTWRITERS);
		h = current->journal_info;
		refcount_inc(&h->use_count);
		WARN_ON(refcount_read(&h->use_count) > 2);
		h->orig_rsv = h->block_rsv;
		h->block_rsv = NULL;
		goto got_it;
	}

	/*
	 * Do the reservation before we join the transaction so we can do all
	 * the appropriate flushing if need be.
	 */
	if (num_items && root != fs_info->chunk_root) {
		struct btrfs_block_rsv *rsv = &fs_info->trans_block_rsv;
		u64 delayed_refs_bytes = 0;

		qgroup_reserved = num_items * fs_info->nodesize;
		ret = btrfs_qgroup_reserve_meta_pertrans(root, qgroup_reserved,
				enforce_qgroups);
		if (ret)
			return ERR_PTR(ret);

		/*
		 * We want to reserve all the bytes we may need all at once, so
		 * we only do 1 enospc flushing cycle per transaction start.  We
		 * accomplish this by simply assuming we'll do 2 x num_items
		 * worth of delayed refs updates in this trans handle, and
		 * refill that amount for whatever is missing in the reserve.
		 */
		num_bytes = btrfs_calc_insert_metadata_size(fs_info, num_items);
		if (flush == BTRFS_RESERVE_FLUSH_ALL &&
		    delayed_refs_rsv->full == 0) {
			delayed_refs_bytes = num_bytes;
			num_bytes <<= 1;
		}

		/*
		 * Do the reservation for the relocation root creation
		 */
		if (need_reserve_reloc_root(root)) {
			num_bytes += fs_info->nodesize;
			reloc_reserved = true;
		}

		ret = btrfs_block_rsv_add(root, rsv, num_bytes, flush);
		if (ret)
			goto reserve_fail;
		if (delayed_refs_bytes) {
			btrfs_migrate_to_delayed_refs_rsv(fs_info, rsv,
							  delayed_refs_bytes);
			num_bytes -= delayed_refs_bytes;
		}

		if (rsv->space_info->force_alloc)
			do_chunk_alloc = true;
	} else if (num_items == 0 && flush == BTRFS_RESERVE_FLUSH_ALL &&
		   !delayed_refs_rsv->full) {
		/*
		 * Some people call with btrfs_start_transaction(root, 0)
		 * because they can be throttled, but have some other mechanism
		 * for reserving space.  We still want these guys to refill the
		 * delayed block_rsv so just add 1 items worth of reservation
		 * here.
		 */
		ret = btrfs_delayed_refs_rsv_refill(fs_info, flush);
		if (ret)
			goto reserve_fail;
	}
again:
	h = kmem_cache_zalloc(btrfs_trans_handle_cachep, GFP_NOFS);
	if (!h) {
		ret = -ENOMEM;
		goto alloc_fail;
	}

	/*
	 * If we are JOIN_NOLOCK we're already committing a transaction and
	 * waiting on this guy, so we don't need to do the sb_start_intwrite
	 * because we're already holding a ref.  We need this because we could
	 * have raced in and did an fsync() on a file which can kick a commit
	 * and then we deadlock with somebody doing a freeze.
	 *
	 * If we are ATTACH, it means we just want to catch the current
	 * transaction and commit it, so we needn't do sb_start_intwrite(). 
	 */
	if (type & __TRANS_FREEZABLE)
		sb_start_intwrite(fs_info->sb);

	if (may_wait_transaction(fs_info, type))
		wait_current_trans(fs_info);

	do {
		ret = join_transaction(fs_info, type);
		if (ret == -EBUSY) {
			wait_current_trans(fs_info);
			if (unlikely(type == TRANS_ATTACH ||
				     type == TRANS_JOIN_NOSTART))
				ret = -ENOENT;
		}
	} while (ret == -EBUSY);

	if (ret < 0)
		goto join_fail;

	cur_trans = fs_info->running_transaction;

	h->transid = cur_trans->transid;
	h->transaction = cur_trans;
	h->root = root;
	refcount_set(&h->use_count, 1);
	h->fs_info = root->fs_info;

	h->type = type;
	h->can_flush_pending_bgs = true;
	INIT_LIST_HEAD(&h->new_bgs);

	smp_mb();
	if (cur_trans->state >= TRANS_STATE_COMMIT_START &&
	    may_wait_transaction(fs_info, type)) {
		current->journal_info = h;
		btrfs_commit_transaction(h);
		goto again;
	}

	if (num_bytes) {
		trace_btrfs_space_reservation(fs_info, "transaction",
					      h->transid, num_bytes, 1);
		h->block_rsv = &fs_info->trans_block_rsv;
		h->bytes_reserved = num_bytes;
		h->reloc_reserved = reloc_reserved;
	}

got_it:
	if (!current->journal_info)
		current->journal_info = h;

	/*
	 * If the space_info is marked ALLOC_FORCE then we'll get upgraded to
	 * ALLOC_FORCE the first run through, and then we won't allocate for
	 * anybody else who races in later.  We don't care about the return
	 * value here.
	 */
	if (do_chunk_alloc && num_bytes) {
		u64 flags = h->block_rsv->space_info->flags;

		btrfs_chunk_alloc(h, btrfs_get_alloc_profile(fs_info, flags),
				  CHUNK_ALLOC_NO_FORCE);
	}

	/*
	 * btrfs_record_root_in_trans() needs to alloc new extents, and may
	 * call btrfs_join_transaction() while we're also starting a
	 * transaction.
	 *
	 * Thus it need to be called after current->journal_info initialized,
	 * or we can deadlock.
	 */
	ret = btrfs_record_root_in_trans(h, root);
	if (ret) {
		/*
		 * The transaction handle is fully initialized and linked with
		 * other structures so it needs to be ended in case of errors,
		 * not just freed.
		 */
		btrfs_end_transaction(h);
		return ERR_PTR(ret);
	}

	return h;

join_fail:
	if (type & __TRANS_FREEZABLE)
		sb_end_intwrite(fs_info->sb);
	kmem_cache_free(btrfs_trans_handle_cachep, h);
alloc_fail:
	if (num_bytes)
		btrfs_block_rsv_release(fs_info, &fs_info->trans_block_rsv,
					num_bytes, NULL);
reserve_fail:
	btrfs_qgroup_free_meta_pertrans(root, qgroup_reserved);
	return ERR_PTR(ret);
}