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.
