mm: rework calculation of bdi_min_ratio in bdi_set_min_ratio

In function bdi_set_min_ratio, min_ratio is unsigned int, it will
result underflow when setting min_ratio below bdi->min_ratio, it
is confusing. Rework it, no functional change.

Link: https://lkml.kernel.org/r/20220422095159.2858305-1-chenwandun@huawei.comSigned-off-by: Chen Wandun <chenwandun@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page-writeback.c

@@ -650,18 +650,25 @@ static unsigned int bdi_min_ratio;
 
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
+	unsigned int delta;
 	int ret = 0;
 
 	spin_lock_bh(&bdi_lock);
 	if (min_ratio > bdi->max_ratio) {
 		ret = -EINVAL;
 	} else {
-		min_ratio -= bdi->min_ratio;
-		if (bdi_min_ratio + min_ratio < 100) {
-			bdi_min_ratio += min_ratio;
-			bdi->min_ratio += min_ratio;
+		if (min_ratio < bdi->min_ratio) {
+			delta = bdi->min_ratio - min_ratio;
+			bdi_min_ratio -= delta;
+			bdi->min_ratio = min_ratio;
 		} else {
-			ret = -EINVAL;
+			delta = min_ratio - bdi->min_ratio;
+			if (bdi_min_ratio + delta < 100) {
+				bdi_min_ratio += delta;
+				bdi->min_ratio = min_ratio;
+			} else {
+				ret = -EINVAL;
+			}
 		}
 	}
 	spin_unlock_bh(&bdi_lock);