Message-ID: <7c12ebaa-4d3d-e475-dfb2-7b459cd26e64@kernel.org>
Date:   Sat, 28 Jan 2023 11:35:34 +0800
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Subject: Re: [PATCH 1/2 v2] f2fs: fix wrong calculation of block age
Content-Language: en-US
To:     qixiaoyu1 <qxy65535@gmail.com>, Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, qixiaoyu1@xiaomi.com,
        xiongping1@xiaomi.com
References: <20230113125859.15651-1-qixiaoyu1@xiaomi.com>
 <20230116030850.20260-1-qixiaoyu1@xiaomi.com>
From:   Chao Yu <chao@kernel.org>
In-Reply-To: <20230116030850.20260-1-qixiaoyu1@xiaomi.com>
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On 2023/1/16 11:08, qixiaoyu1 wrote:
> Currently we wrongly calculate the new block age to
> old * LAST_AGE_WEIGHT / 100.
> 
> Fix it to new * (100 - LAST_AGE_WEIGHT) / 100
>                  + old * LAST_AGE_WEIGHT / 100.
> 
> Signed-off-by: qixiaoyu1 <qixiaoyu1@xiaomi.com>
> Signed-off-by: xiongping1 <xiongping1@xiaomi.com>
> ---
> Change log v1 -> v2:
>   - fix udiv
> 
>   fs/f2fs/extent_cache.c | 7 ++-----
>   1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c
> index 342af24b2f8c..ad5533f178fd 100644
> --- a/fs/f2fs/extent_cache.c
> +++ b/fs/f2fs/extent_cache.c
> @@ -874,11 +874,8 @@ void f2fs_update_read_extent_tree_range_compressed(struct inode *inode,
>   static unsigned long long __calculate_block_age(unsigned long long new,
>   						unsigned long long old)
>   {
> -	unsigned long long diff;
> -
> -	diff = (new >= old) ? new - (new - old) : new + (old - new);
> -
> -	return div_u64(diff * LAST_AGE_WEIGHT, 100);
> +	return div_u64(new, 100) * (100 - LAST_AGE_WEIGHT)
> +		+ div_u64(old, 100) * LAST_AGE_WEIGHT;

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,

>   }
>   
>   /* This returns a new age and allocated blocks in ei */