Re: [PATCH] scsi: Update max_hw_sectors on rescan

[Brian King <brking@linux.vnet.ibm.com>]

On 1/18/24 9:44 AM, John Garry wrote:
> On 17/01/2024 21:36, Brian King wrote:
>> This addresses an issue discovered on ibmvfc LUNs. For this driver,
>> max_sectors is negotiated with the VIOS. This gets done at initialization
>> time, then LUNs get scanned and things generally work fine. However,
>> this attribute can be changed on the VIOS, either due to a sysadmin
>> change or potentially a VIOS code level change. If this decreases
>> to a smaller value, due to one of these reasons, the next time the
>> ibmvfc driver performs an NPIV login, it will only be able to use
>> the smaller value. In the case of a VIOS reboot, when the VIOS goes
>> down, all paths through that VIOS will go to devloss state. When
>> the VIOS comes back up, ibmvfc negotiates max_sectors and will only
>> be able to get the smaller value and it will update shost->max_sectors.
> 
> Are you saying that the driver will manually update shost->max_sectors after adding the scsi host? I didn't think that was permitted.

That is what happens. The characteristics of the underlying hardware can change across
a virtual adapter reset. 

Thanks,

Brian

> 
> Thanks,
> John
> 
>> However, when LUNs are scanned, the devloss paths will be found
>> and brought back online, still using the old max_hw_sectors. This
>> change ensures that max_hw_sectors gets updated.
>>
>> Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
>> ---
>>   drivers/scsi/scsi_scan.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/scsi/scsi_scan.c b/drivers/scsi/scsi_scan.c
>> index 44680f65ea14..01f2b38daab3 100644
>> --- a/drivers/scsi/scsi_scan.c
>> +++ b/drivers/scsi/scsi_scan.c
>> @@ -1162,6 +1162,7 @@ static int scsi_probe_and_add_lun(struct scsi_target *starget,
>>       blist_flags_t bflags;
>>       int res = SCSI_SCAN_NO_RESPONSE, result_len = 256;
>>       struct Scsi_Host *shost = dev_to_shost(starget->dev.parent);
>> +    struct request_queue *q;
>>         /*
>>        * The rescan flag is used as an optimization, the first scan of a
>> @@ -1182,6 +1183,10 @@ static int scsi_probe_and_add_lun(struct scsi_target *starget,
>>                   *bflagsp = scsi_get_device_flags(sdev,
>>                                    sdev->vendor,
>>                                    sdev->model);
>> +            q = sdev->request_queue;
>> +            if (queue_max_hw_sectors(q) > shost->max_sectors)
>> +                blk_queue_max_hw_sectors(q, shost->max_sectors);
>> +
>>               return SCSI_SCAN_LUN_PRESENT;
>>           }
>>           scsi_device_put(sdev);
>> @@ -2006,4 +2011,3 @@ void scsi_forget_host(struct Scsi_Host *shost)
>>       }
>>       spin_unlock_irqrestore(shost->host_lock, flags);
>>   }
>> -
> 

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center