Re: [PATCH v2 1/3] scsi_cmnd: Introduce scsi_transfer_length helper

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

On 6/24/2014 7:08 PM, Michael Christie wrote:
> On Jun 24, 2014, at 7:53 AM, Martin K. Petersen <martin.petersen@oracle.com> wrote:
>
>>>>>>> "Mike" == Mike Christie <michaelc@cs.wisc.edu> writes:
>> Mike> The problem is WRITE_SAME requests are setup so that
>> Mike> req->__data_len is the value of the entire request when the setup
>> Mike> is completed but during the setup process it's value changes
>>
>> Oh, I see. So things break because iSCSI uses scsi_transfer_length()
>> where the scatterlist length was used in the past.
>>
>> How about this?
>>
>>
>> SCSI: Use SCSI data buffer length to extract transfer size
>>
>> Commit 8846bab180fa introduced a helper that can be used to query the
>> wire transfer size for a SCSI command taking protection information into
>> account.
>>
>> However, some commands do not have a 1:1 mapping between the block range
>> they work on and the payload size (discard, write same). After the
>> scatterlist has been set up these requests use __data_len to store the
>> number of bytes to report completion on. This means that callers of
>> scsi_transfer_length() would get the wrong byte count for these types of
>> requests.
>>
>> To overcome this we make scsi_transfer_length() use the scatterlist
>> length in the scsi_data_buffer as basis for the wire transfer
>> calculation instead of __data_len.
>>
>> Reported-by: Christoph Hellwig <hch@infradead.org>
>> Debugged-by: Mike Christie <michaelc@cs.wisc.edu>
>> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
>>
>> diff --git a/include/scsi/scsi_cmnd.h b/include/scsi/scsi_cmnd.h
>> index 42ed789ebafc..e0ae71098144 100644
>> --- a/include/scsi/scsi_cmnd.h
>> +++ b/include/scsi/scsi_cmnd.h
>> @@ -318,7 +318,7 @@ static inline void set_driver_byte(struct scsi_cmnd *cmd, char status)
>>
>> static inline unsigned scsi_transfer_length(struct scsi_cmnd *scmd)
>> {
>> -	unsigned int xfer_len = blk_rq_bytes(scmd->request);
>> +	unsigned int xfer_len = scsi_out(scmd)->length;
>> 	unsigned int prot_op = scsi_get_prot_op(scmd);
>> 	unsigned int sector_size = scmd->device->sector_size;
>
> Do we need to check for the data direction. Something like
>
> if (scmd->sc_data_direction == DMA_TO_DEVICE)
> 	xfer_len = scsi_out(scmnd)->length;
> else
> 	xfer_len = scsi_in(scmnd)->length;

This condition only matters in the bidi case, which is not relevant for 
the PI case.
I suggested to condition that in libiscsi (posted in the second thread, 
copy-paste below).
Although I do agree that scsi_transfer_length() helper is not really 
just for PI and not more.
I think Mike's way is cleaner.


diff --git a/drivers/scsi/libiscsi.c b/drivers/scsi/libiscsi.c
index 3f46234..abf0c3e 100644
--- a/drivers/scsi/libiscsi.c
+++ b/drivers/scsi/libiscsi.c
@@ -386,12 +386,14 @@ static int iscsi_prep_scsi_cmd_pdu(struct 
iscsi_task *task)
                 rc = iscsi_prep_bidi_ahs(task);
                 if (rc)
                         return rc;
+               transfer_length = scsi_in(sc)->length;
+       } else {
+               transfer_length = scsi_transfer_length(sc);
         }

         if (scsi_get_prot_op(sc) != SCSI_PROT_NORMAL)
                 task->protected = true;

-       transfer_length = scsi_transfer_length(sc);
         hdr->data_length = cpu_to_be32(transfer_length);
         if (sc->sc_data_direction == DMA_TO_DEVICE) {
                 struct iscsi_r2t_info *r2t = &task->unsol_r2t;

Sagi.