static int read_one_block_group(struct btrfs_fs_info *info,
				struct btrfs_block_group_item *bgi,
				const struct btrfs_key *key,
				int need_clear)
{
	struct btrfs_block_group *cache;
	struct btrfs_space_info *space_info;
	const bool mixed = btrfs_fs_incompat(info, MIXED_GROUPS);
	int ret;

	ASSERT(key->type == BTRFS_BLOCK_GROUP_ITEM_KEY);

	cache = btrfs_create_block_group_cache(info, key->objectid);
	if (!cache)
		return -ENOMEM;

	cache->length = key->offset;
	cache->used = btrfs_stack_block_group_used(bgi);
	cache->flags = btrfs_stack_block_group_flags(bgi);

	set_free_space_tree_thresholds(cache);

	if (need_clear) {
		/*
		 * When we mount with old space cache, we need to
		 * set BTRFS_DC_CLEAR and set dirty flag.
		 *
		 * a) Setting 'BTRFS_DC_CLEAR' makes sure that we
		 *    truncate the old free space cache inode and
		 *    setup a new one.
		 * b) Setting 'dirty flag' makes sure that we flush
		 *    the new space cache info onto disk.
		 */
		if (btrfs_test_opt(info, SPACE_CACHE))
			cache->disk_cache_state = BTRFS_DC_CLEAR;
	}
	if (!mixed && ((cache->flags & BTRFS_BLOCK_GROUP_METADATA) &&
	    (cache->flags & BTRFS_BLOCK_GROUP_DATA))) {
			btrfs_err(info,
"bg %llu is a mixed block group but filesystem hasn't enabled mixed block groups",
				  cache->start);
			ret = -EINVAL;
			goto error;
	}

	ret = btrfs_load_block_group_zone_info(cache, false);
	if (ret) {
		btrfs_err(info, "zoned: failed to load zone info of bg %llu",
			  cache->start);
		goto error;
	}

	/*
	 * We need to exclude the super stripes now so that the space info has
	 * super bytes accounted for, otherwise we'll think we have more space
	 * than we actually do.
	 */
	ret = exclude_super_stripes(cache);
	if (ret) {
		/* We may have excluded something, so call this just in case. */
		btrfs_free_excluded_extents(cache);
		goto error;
	}

	/*
	 * For zoned filesystem, space after the allocation offset is the only
	 * free space for a block group. So, we don't need any caching work.
	 * btrfs_calc_zone_unusable() will set the amount of free space and
	 * zone_unusable space.
	 *
	 * For regular filesystem, check for two cases, either we are full, and
	 * therefore don't need to bother with the caching work since we won't
	 * find any space, or we are empty, and we can just add all the space
	 * in and be done with it.  This saves us _a_lot_ of time, particularly
	 * in the full case.
	 */
	if (btrfs_is_zoned(info)) {
		btrfs_calc_zone_unusable(cache);
	} else if (cache->length == cache->used) {
		cache->last_byte_to_unpin = (u64)-1;
		cache->cached = BTRFS_CACHE_FINISHED;
		btrfs_free_excluded_extents(cache);
	} else if (cache->used == 0) {
		cache->last_byte_to_unpin = (u64)-1;
		cache->cached = BTRFS_CACHE_FINISHED;
		add_new_free_space(cache, cache->start,
				   cache->start + cache->length);
		btrfs_free_excluded_extents(cache);
	}

	ret = btrfs_add_block_group_cache(info, cache);
	if (ret) {
		btrfs_remove_free_space_cache(cache);
		goto error;
	}
	trace_btrfs_add_block_group(info, cache, 0);
	btrfs_update_space_info(info, cache->flags, cache->length,
				cache->used, cache->bytes_super,
				cache->zone_unusable, &space_info);

	cache->space_info = space_info;

	link_block_group(cache);

	set_avail_alloc_bits(info, cache->flags);
	if (btrfs_chunk_readonly(info, cache->start)) {
		inc_block_group_ro(cache, 1);
	} else if (cache->used == 0) {
		ASSERT(list_empty(&cache->bg_list));
		if (btrfs_test_opt(info, DISCARD_ASYNC))
			btrfs_discard_queue_work(&info->discard_ctl, cache);
		else
			btrfs_mark_bg_unused(cache);
	}
	return 0;
error:
	btrfs_put_block_group(cache);
	return ret;
}