linux-kernel - Re: [PATCH 1/2 v2] f2fs: fix wrong calculation of block age

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Message-ID: <938a8e61-4e47-1acc-938c-c90d213d2c86@kernel.org>
Date:   Wed, 1 Feb 2023 20:57:33 +0800
From:   Chao Yu <chao@...nel.org>
To:     qixiaoyu <qxy65535@...il.com>, Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     xiongping1@...omi.com, qixiaoyu1@...omi.com,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH 1/2 v2] f2fs: fix wrong calculation of block age

On 2023/2/1 20:23, qixiaoyu wrote:
>>>
>>> How about updating as below to avoid lossing accuracy if new is less than 100?
>>>
>>> return div_u64(new * (100 - LAST_AGE_WEIGHT), 100) +
>>> 		div_u64(old * LAST_AGE_WEIGHT, 100);
>>>
>>> Thanks,
>>>
>>
>> We want to avoid overflow by doing the division first. To keep the accuracy, how

Alright,

>> about updating as below:
>>
>> 	res = div_u64_rem(new, 100, &rem_new) * (100 - LAST_AGE_WEIGHT)
>> 		+ div_u64_rem(old, 100, &rem_old) * LAST_AGE_WEIGHT;
>> 	res += rem_new * (100 - LAST_AGE_WEIGHT) / 100 + rem_old * LAST_AGE_WEIGHT / 100;
>> 	return res;

if (rem_new)
	res += rem_new * (100 - LAST_AGE_WEIGHT) / 100;
if (rem_old)
	res += rem_old * LAST_AGE_WEIGHT / 100;

Otherwise, it looks fine to me. :)

Thanks,

>>
>> Thanks,
>>
> 
> Friendly ping
> 
>>>>    }
>>>>    /* This returns a new age and allocated blocks in ei */