From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH 3/5] s390, dcssblk: Allow a NULL-kaddr to ->direct_access() References: <20180724084510.6104-1-yehs2007@zoho.com> <20180724084510.6104-4-yehs2007@zoho.com> From: Christian Borntraeger Date: Tue, 24 Jul 2018 10:53:39 +0200 MIME-Version: 1.0 In-Reply-To: <20180724084510.6104-4-yehs2007@zoho.com> Content-Language: en-US Message-Id: <192a9dac-921f-e222-3a7d-32d43679cd12@de.ibm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-fsdevel-owner@vger.kernel.org List-Archive: List-Post: To: Huaisheng Ye , linux-nvdimm@lists.01.org, dan.j.williams@intel.com Cc: ross.zwisler@linux.intel.com, willy@infradead.org, vishal.l.verma@intel.com, dave.jiang@intel.com, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, viro@zeniv.linux.org.uk, martin.petersen@oracle.com, axboe@kernel.dk, gregkh@linuxfoundation.org, bart.vanassche@wdc.com, jack@suse.cz, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux-fsdevel@vger.kernel.org, chengnt@lenovo.com, Huaisheng Ye List-ID: On 07/24/2018 10:45 AM, Huaisheng Ye wrote: > From: Huaisheng Ye > > 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 > --- > 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. > *pfn = __pfn_to_pfn_t(PFN_DOWN(dev_info->start + offset), > PFN_DEV|PFN_SPECIAL); > >