linux-kernel - RE: [External] Re: [PATCH 3/5] s390, dcssblk: Allow a NULL-kaddr to ->direct

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Date:   Tue, 24 Jul 2018 09:46:43 +0000
From:   Huaisheng HS1 Ye <yehs1@...ovo.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>
CC:     "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
        "heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "bart.vanassche@....com" <bart.vanassche@....com>,
        "jack@...e.cz" <jack@...e.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        NingTing Cheng <chengnt@...ovo.com>,
        Huaisheng Ye <yehs2007@...o.com>
Subject: RE: [External]  Re: [PATCH 3/5] s390, dcssblk: Allow a NULL-kaddr to
 ->direct_access()

From: Christian Borntraeger <borntraeger@...ibm.com>
Sent: Tuesday, July 24, 2018 4:54 PM
> On 07/24/2018 10:45 AM, Huaisheng Ye wrote:
> > From: Huaisheng Ye <yehs1@...ovo.com>
> >
> > 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@...ovo.com>
> > ---
> >  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.

Cheers,
Huaisheng Ye