linux-kernel - [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max

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Date: Tue, 19 Dec 2023 10:42:46 +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,
	linux-fsdevel@...r.kernel.org,
	linux-block@...r.kernel.org
Subject: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio

Since now bdi->max_ratio is part per million, fix the wrong arithmetic
for max_prop_frac when setting max_ratio.  Otherwise the miscalculated
max_prop_frac will affect the incrementing of writeout completion count
when max_ratio is not 100%.

Fixes: efc3e6ad53ea ("mm: split off __bdi_set_max_ratio() function")
Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>
---
 mm/page-writeback.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2140382dd768..dda59b368c01 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -728,7 +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,
+					       100 * BDI_RATIO_SCALE);
 	}
 	spin_unlock_bh(&bdi_lock);
 
-- 
2.19.1.6.gb485710b