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Subject: Re: [PATCH] bcache: remove the redundant judgment on bi_size
To:     Xifengfei <xi.fengfei@h3c.com>
Cc:     "linux-bcache@vger.kernel.org" <linux-bcache@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kent.overstreet@gmail.com" <kent.overstreet@gmail.com>
References: <ed49f33da50e4f5c88f986c8ed239e78@h3c.com>
From:   Coly Li <colyli@suse.de>
Message-ID: <51b8745f-a41e-a8cb-7921-847bcee4a157@suse.de>
Date:   Tue, 31 Aug 2021 09:35:09 +0800
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On 8/30/21 2:29 PM, Xifengfei wrote:
> （Sorry, there was an obvious typo in the last email）
> Thanks a lot. I understand the purpose.
> So is the original judgment process too complicated?  Can we judge bi_size directly?  This will be more concise
>
> @@ -423,7 +423,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
>          add_sequential(task);
>          i->sequential = 0;
>   found:
> -       if (i->sequential + bio->bi_iter.bi_size > i->sequential)
> +       if (bio->bi_iter.bi_size)
>                  i->sequential   += bio->bi_iter.bi_size;
>
>          i->last                  = bio_end_sector(bio);

The above change works, but the code readability decreased because 
how/why i->sequential is maintained is not that directly visible.

This is a difference of coding styles. IMHO for this particular case, 
the readability is more important than less CPU instructions.

Thanks.

Coly Li

> Thanks
> Fengfei
>
> -----邮件原件-----
> 发件人: Coly Li [mailto:colyli@suse.de]
> 发送时间: 2021年8月29日 15:50
> 收件人: xifengfei (RD) <xi.fengfei@h3c.com>
> 抄送: linux-bcache@vger.kernel.org; linux-kernel@vger.kernel.org; kent.overstreet@gmail.com
> 主题: Re: [PATCH] bcache: remove the redundant judgment on bi_size
>
> On 8/29/21 12:49 PM, Fengfei Xi wrote:
>> The bi_size is unsigned int type data not less than 0, so we can
>> directly add bi_size without extra judgment
>>
>> Signed-off-by: Fengfei Xi <xi.fengfei@h3c.com>
> NACK. The check is necessary to avoid redundant and unnecessary memory write.
>
> Coly Li
>
>> ---
>>    drivers/md/bcache/request.c | 4 +---
>>    1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
>> index 6d1de889b..2788eec3a 100644
>> --- a/drivers/md/bcache/request.c
>> +++ b/drivers/md/bcache/request.c
>> @@ -423,9 +423,7 @@ static bool check_should_bypass(struct cached_dev *dc, struct bio *bio)
>>    	add_sequential(task);
>>    	i->sequential = 0;
>>    found:
>> -	if (i->sequential + bio->bi_iter.bi_size > i->sequential)
>> -		i->sequential	+= bio->bi_iter.bi_size;
>> -
>> +	i->sequential		+= bio->bi_iter.bi_size;
>>    	i->last			 = bio_end_sector(bio);
>>    	i->jiffies		 = jiffies + msecs_to_jiffies(5000);
>>    	task->sequential_io	 = i->sequential;