[PATCH v2] nvmet-tcp: fix data digest calculation for multiple H2CData PDUs

[Varun Prakash]

>> +{
>> +	struct scatterlist *sg = &cmd->req.sg[cmd->sg_idx];
>> +	struct bio_vec *iov = cmd->iov;
>> +	u32 data_length = cmd->pdu_len;
>> +
>> +	crypto_ahash_init(hash);
>> +
>> +	if (iov->bv_offset) {
>> +		struct scatterlist first_sg;
>> +
>> +		sg_init_table(&first_sg, 1);
>> +		sg_set_page(&first_sg, sg_page(sg), iov->bv_len,
>> +				iov->bv_offset);
>> +
>> +		data_length -= iov->bv_len;
>> +
>> +		if (data_length) {
>> +			ahash_request_set_crypt(hash, &first_sg, NULL,
>> +						iov->bv_len);
>> +			crypto_ahash_update(hash);
>> +			sg = sg_next(sg);
>> +		} else {
>> +			ahash_request_set_crypt(hash, &first_sg,
>> +					(void *)&cmd->exp_ddgst, iov->bv_len);
>> +			crypto_ahash_finup(hash);
>> +			return;
>> +		}
>
>What is the case for data_length=0 ? I'm not entirely clear how this can
>happen.

This can happen if H2CData PDU data length is not a multiple of target page size.

For example if transfer length is 32K, page size is 4K and H2CData PDU data length is 6K,
host will send 6 H2CData PDUs, for the last PDU iov->bv_offset = 2K,
iov->bv_len = 2K and data_length = 2K.