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

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Date:   Tue, 24 Jul 2018 10:53:39 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Huaisheng Ye <yehs2007@...o.com>, linux-nvdimm@...ts.01.org,
        dan.j.williams@...el.com
Cc:     ross.zwisler@...ux.intel.com, willy@...radead.org,
        vishal.l.verma@...el.com, dave.jiang@...el.com,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        viro@...iv.linux.org.uk, martin.petersen@...cle.com,
        axboe@...nel.dk, gregkh@...uxfoundation.org,
        bart.vanassche@....com, jack@...e.cz, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        chengnt@...ovo.com, Huaisheng Ye <yehs1@...ovo.com>
Subject: Re: [PATCH 3/5] s390, dcssblk: Allow a NULL-kaddr to
 ->direct_access()



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.


>  	*pfn = __pfn_to_pfn_t(PFN_DOWN(dev_info->start + offset),
>  			PFN_DEV|PFN_SPECIAL);
>  
>