Date:   Wed, 1 Feb 2023 20:23:04 +0800
From:   qixiaoyu <qxy65535@gmail.com>
To:     Chao Yu <chao@kernel.org>, Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     xiongping1@xiaomi.com, qixiaoyu1@xiaomi.com,
        linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [PATCH 1/2 v2] f2fs: fix wrong calculation of block age
Message-ID: <Y9pZqOMBipT2NZk0@mi-HP-ProDesk-680-G4-MT>
References: <20230113125859.15651-1-qixiaoyu1@xiaomi.com>
 <20230116030850.20260-1-qixiaoyu1@xiaomi.com>
 <7c12ebaa-4d3d-e475-dfb2-7b459cd26e64@kernel.org>
 <Y9ZWDVV3HJ431Fis@mi-HP-ProDesk-680-G4-MT>
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> > 
> > 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
> 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;
> 
> Thanks,
> 

Friendly ping

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