Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio

From: Jingbo Xu <jefflexu@linux.alibaba.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: shr@devkernel.io, akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, joseph.qi@linux.alibaba.com,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio
Date: Tue, 19 Dec 2023 22:07:12 +0800	[thread overview]
Message-ID: <c194c632-5bf9-40ec-ab3b-6ebbd9f199fa@linux.alibaba.com> (raw)
In-Reply-To: <ZYGUOslxxwe1sNzR@casper.infradead.org>

On 12/19/23 9:01 PM, Matthew Wilcox wrote:
> On Tue, Dec 19, 2023 at 01:58:21PM +0800, Jingbo Xu wrote:
>> On 12/19/23 12:06 PM, Matthew Wilcox wrote:
>>> On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote:
>>>>  	} else {
>>>>  		bdi->max_ratio = max_ratio;
>>>> -		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
>>>> +		bdi->max_prop_frac = div64_u64(FPROP_FRAC_BASE * max_ratio,
>>>> +					       100 * BDI_RATIO_SCALE);
>>>>  	}
>>>
>>> Why use div64_u64 here?
>>>
>>> FPROP_FRAC_BASE is an unsigned long.  max_ratio is an unsigned int, so
>>> the numerator is an unsigned long.  BDI_RATIO_SCALE is 10,000, so the
>>> numerator is an unsigned int.  There's no 64-bit arithmetic needed here.
>>
>> Yes, div64_u64() is actually not needed here. So it seems
>>
>> bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE;
>>
>> is adequate?
> 
> I'd rather spell that as:
> 
> 		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
> 					(100 * BDI_RATIO_SCALE);
> 
> It's closer to how you'd write it out mathematically and so it reads
> more easily.  At least for me.

Thanks, I would send v3 soon.

-- 
Thanks,
Jingbo