Re: [External] Re: [PATCH 3/5] s390, dcssblk: Allow a NULL-kaddr to ->direct_access()

[Christian Borntraeger]

On 07/24/2018 11:46 AM, Huaisheng HS1 Ye wrote:
> From: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> Sent: Tuesday, July 24, 2018 4:54 PM
>> On 07/24/2018 10:45 AM, Huaisheng Ye wrote:
>>> From: Huaisheng Ye <yehs1@xxxxxxxxxx>
>>>
>>> dcssblk_direct_access() needs to check the validity of second rank
>>> pointer kaddr for NULL assignment. If kaddr equals to NULL, it
>>> doesn't need to calculate the value.
>>>
>>> Signed-off-by: Huaisheng Ye <yehs1@xxxxxxxxxx>
>>> ---
>>>  drivers/s390/block/dcssblk.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
>>> index 0a312e4..9c13dc5 100644
>>> --- a/drivers/s390/block/dcssblk.c
>>> +++ b/drivers/s390/block/dcssblk.c
>>> @@ -915,7 +915,8 @@ static DEVICE_ATTR(save, S_IWUSR | S_IRUSR, dcssblk_save_show,
>>>  	unsigned long dev_sz;
>>>
>>>  	dev_sz = dev_info->end - dev_info->start + 1;
>>> -	*kaddr = (void *) dev_info->start + offset;
>>> +	if (kaddr)
>>> +		*kaddr = (void *) dev_info->start + offset;
>>
>> So you are trading of a load + add (dev_info->start should be cache hot) against a
>> compare+branch . Not sure that this is always a win.
> 
> Hmm...the calculation process of pfn is more complicated than kaddr. I think you agree to check pfn but not sure kaddr, right?
> From the logical consistency of code, I think it shall be better to give pfn and kaddr similar treatment.

Reading it again, its more that I do not like the patch description. It reads
like an optimization, (and I think it is not) but it should rather read more
like "with an upcoming change kaddr can be NULL" or so.