From:   qixiaoyu1 <qxy65535@gmail.com>
To:     Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <chao@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        Ping Xiong <xiongping1@xiaomi.com>,
        Xiaoyu Qi <qixiaoyu1@xiaomi.com>
Subject: [PATCH 1/2 v3] f2fs: fix wrong calculation of block age
Date:   Thu,  2 Feb 2023 16:20:27 +0800
Message-Id: <20230202082028.9013-1-qixiaoyu1@xiaomi.com>
In-Reply-To: <938a8e61-4e47-1acc-938c-c90d213d2c86@kernel.org>
References: <938a8e61-4e47-1acc-938c-c90d213d2c86@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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
Change log v2 -> v3:
 - keep the accuracy with div_u64_rem

 fs/f2fs/extent_cache.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/extent_cache.c b/fs/f2fs/extent_cache.c
index 342af24b2f8c..d9f12f404beb 100644
--- a/fs/f2fs/extent_cache.c
+++ b/fs/f2fs/extent_cache.c
@@ -874,11 +874,18 @@ 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;
+	unsigned int rem_old, rem_new;
+	unsigned long long res;
 
-	diff = (new >= old) ? new - (new - old) : new + (old - new);
+	res = div_u64_rem(new, 100, &rem_new) * (100 - LAST_AGE_WEIGHT)
+		+ div_u64_rem(old, 100, &rem_old) * LAST_AGE_WEIGHT;
 
-	return div_u64(diff * LAST_AGE_WEIGHT, 100);
+	if (rem_new)
+		res += rem_new * (100 - LAST_AGE_WEIGHT) / 100;
+	if (rem_old)
+		res += rem_old * LAST_AGE_WEIGHT / 100;
+
+	return res;
 }
 
 /* This returns a new age and allocated blocks in ei */
-- 
2.39.0