noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
			      u64 *orig_start, u64 *orig_block_len,
			      u64 *ram_bytes)
{
	struct btrfs_trans_handle *trans;
	struct btrfs_path *path;
	int ret;
	struct extent_buffer *leaf;
	struct btrfs_root *root = BTRFS_I(inode)->root;
	struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
	struct btrfs_file_extent_item *fi;
	struct btrfs_key key;
	u64 disk_bytenr;
	u64 backref_offset;
	u64 extent_end;
	u64 num_bytes;
	int slot;
	int found_type;
	bool nocow = (BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW);

	path = btrfs_alloc_path();
	if (!path)
		return -ENOMEM;

	ret = btrfs_lookup_file_extent(NULL, root, path, btrfs_ino(inode),
				       offset, 0);
	if (ret < 0)
		goto out;

	slot = path->slots[0];
	if (ret == 1) {
		if (slot == 0) {
			/* can't find the item, must cow */
			ret = 0;
			goto out;
		}
		slot--;
	}
	ret = 0;
	leaf = path->nodes[0];
	btrfs_item_key_to_cpu(leaf, &key, slot);
	if (key.objectid != btrfs_ino(inode) ||
	    key.type != BTRFS_EXTENT_DATA_KEY) {
		/* not our file or wrong item type, must cow */
		goto out;
	}

	if (key.offset > offset) {
		/* Wrong offset, must cow */
		goto out;
	}

	fi = btrfs_item_ptr(leaf, slot, struct btrfs_file_extent_item);
	found_type = btrfs_file_extent_type(leaf, fi);
	if (found_type != BTRFS_FILE_EXTENT_REG &&
	    found_type != BTRFS_FILE_EXTENT_PREALLOC) {
		/* not a regular extent, must cow */
		goto out;
	}

	if (!nocow && found_type == BTRFS_FILE_EXTENT_REG)
		goto out;

	extent_end = key.offset + btrfs_file_extent_num_bytes(leaf, fi);
	if (extent_end <= offset)
		goto out;

	disk_bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
	if (disk_bytenr == 0)
		goto out;

	if (btrfs_file_extent_compression(leaf, fi) ||
	    btrfs_file_extent_encryption(leaf, fi) ||
	    btrfs_file_extent_other_encoding(leaf, fi))
		goto out;

	backref_offset = btrfs_file_extent_offset(leaf, fi);

	if (orig_start) {
		*orig_start = key.offset - backref_offset;
		*orig_block_len = btrfs_file_extent_disk_num_bytes(leaf, fi);
		*ram_bytes = btrfs_file_extent_ram_bytes(leaf, fi);
	}

	if (btrfs_extent_readonly(root, disk_bytenr))
		goto out;

	num_bytes = min(offset + *len, extent_end) - offset;
	if (!nocow && found_type == BTRFS_FILE_EXTENT_PREALLOC) {
		u64 range_end;

		range_end = round_up(offset + num_bytes, root->sectorsize) - 1;
		ret = test_range_bit(io_tree, offset, range_end,
				     EXTENT_DELALLOC, 0, NULL);
		if (ret) {
			ret = -EAGAIN;
			goto out;
		}
	}

	btrfs_release_path(path);

	/*
	 * look for other files referencing this extent, if we
	 * find any we must cow
	 */
	trans = btrfs_join_transaction(root);
	if (IS_ERR(trans)) {
		ret = 0;
		goto out;
	}

	ret = btrfs_cross_ref_exist(trans, root, btrfs_ino(inode),
				    key.offset - backref_offset, disk_bytenr);
	btrfs_end_transaction(trans, root);
	if (ret) {
		ret = 0;
		goto out;
	}

	/*
	 * adjust disk_bytenr and num_bytes to cover just the bytes
	 * in this extent we are about to write.  If there
	 * are any csums in that range we have to cow in order
	 * to keep the csums correct
	 */
	disk_bytenr += backref_offset;
	disk_bytenr += offset - key.offset;
	if (csum_exist_in_range(root, disk_bytenr, num_bytes))
				goto out;
	/*
	 * all of the above have passed, it is safe to overwrite this extent
	 * without cow
	 */
	*len = num_bytes;
	ret = 1;
out:
	btrfs_free_path(path);
	return ret;
}