linux-kernel - [PATCH] mm: fix arithmetic for bdi min_ratio and max

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Date: Mon, 18 Dec 2023 11:16:40 +0800
From: Jingbo Xu <jefflexu@...ux.alibaba.com>
To: shr@...kernel.io,
	akpm@...ux-foundation.org
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	joseph.qi@...ux.alibaba.com
Subject: [PATCH] mm: fix arithmetic for bdi min_ratio and max_ratio

bdi->[min|max]_ratio are both part per million.  Fix the wrong
arithmetic when setting bdi's min_ratio and max_ratio.

Fixes: efc3e6ad53ea ("mm: split off __bdi_set_max_ratio() function")
Fixes: 8021fb3232f2 ("mm: split off __bdi_set_min_ratio() function")
Reported-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>
---
 mm/page-writeback.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index ee2fd6a6af40..b393b3281ce9 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -692,7 +692,6 @@ static int __bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ra
 
 	if (min_ratio > 100 * BDI_RATIO_SCALE)
 		return -EINVAL;
-	min_ratio *= BDI_RATIO_SCALE;
 
 	spin_lock_bh(&bdi_lock);
 	if (min_ratio > bdi->max_ratio) {
@@ -729,7 +728,8 @@ static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ra
 		ret = -EINVAL;
 	} 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,
+					       100UL * BDI_RATIO_SCALE);
 	}
 	spin_unlock_bh(&bdi_lock);
 
-- 
2.19.1.6.gb485710b