Re: [PATCH v2] mpt3sas: Force request partial completion alignment

["Guilherme G. Piccoli" <gpiccoli@linux.vnet.ibm.com>]

On 01/25/2017 09:46 PM, Martin K. Petersen wrote:
>>>>>> "Guilherme" == Guilherme G Piccoli <gpiccoli@linux.vnet.ibm.com> writes:
> 
> Hi Guilherme,

Hi Martin, thanks for the review!


> 
> diff --git a/drivers/scsi/mpt3sas/mpt3sas_scsih.c b/drivers/scsi/mpt3sas/mpt3sas_scsih.c
> index 75f3fce..e52c942 100644
> --- a/drivers/scsi/mpt3sas/mpt3sas_scsih.c
> +++ b/drivers/scsi/mpt3sas/mpt3sas_scsih.c
> @@ -4657,6 +4657,8 @@ _scsih_io_done(struct MPT3SAS_ADAPTER *ioc, u16 smid, u8 msix_index, u32 reply)
>  	struct MPT3SAS_DEVICE *sas_device_priv_data;
>  	u32 response_code = 0;
>  	unsigned long flags;
> +	unsigned int sector_sz;
> +	struct request *req;
> 
>  	mpi_reply = mpt3sas_base_get_reply_virt_addr(ioc, reply);
>  	scmd = _scsih_scsi_lookup_get_clear(ioc, smid);
> @@ -4715,6 +4717,21 @@ _scsih_io_done(struct MPT3SAS_ADAPTER *ioc, u16 smid, u8 msix_index, u32 reply)
>  	}
> 
>  	xfer_cnt = le32_to_cpu(mpi_reply->TransferCount);
> +
> +	/* In case of bogus fw or device, we could end up having
> +	 * unaligned partial completion. We can force alignment here,
> +	 * then scsi-ml does not need to handle this misbehavior.
> +	 */
> +	sector_sz = scmd->device->sector_size;
> +	req = scmd->request;
> +	if (unlikely(sector_sz && req && (req->cmd_type == REQ_TYPE_FS) &&
> +		    (xfer_cnt % sector_sz))) {
> 
> Maybe a bit zealous on the sanity checking...

A bit...? heheh
Too much I'd say. Since this is dealing with a bogus FW scenario, I
found more safe to check everything...of course we can remove checks if
it's sure req isn't NULL ever. The sector_sz check is avoiding
degenerate cases, since our division below.


> 
> +		sdev_printk(KERN_INFO, scmd->device,
> +			"unaligned partial completion avoided (xfer_cnt=%u, sector_sz=%u)\n",
> +			xfer_cnt, sector_sz);
> +		xfer_cnt = (xfer_cnt / sector_sz) * sector_sz;
> 
> Not so keen on divisions. xfer_cnt = round_down(xfer_cnt, sector_sz), maybe?
>

Martin, I might be completely wrong here (please correct me if this is
the case), but isn't C standard integer division a truncation that acts
like a round down? I checked (what I think is) the specification of C
language (ISO/IEC 9899:1999), and it seems the division proposed by Ram
Pai is accurate in this case. Also, both variables are unsigned.

Let me know your thoughts.
Thanks,


Guilherme