blk-iocost: fix divide-by-zero in transfer_surpluses()

Conceptually, root_iocg->hweight_donating must be less than WEIGHT_ONE but
all hweight calculations round up and thus it may end up >= WEIGHT_ONE
triggering divide-by-zero and other issues. Bound the value to avoid
surprises.

Fixes: e08d02aa ("blk-iocost: implement Andy's method for donation weight updates")
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -1881,15 +1881,21 @@ static void transfer_surpluses(struct list_head *surpluses, struct ioc_now *now)
 
 	/*
 	 * Calculate the global donation rate (gamma) - the rate to adjust
-	 * non-donating budgets by. No need to use 64bit multiplication here as
-	 * the first operand is guaranteed to be smaller than WEIGHT_ONE
-	 * (1<<16).
+	 * non-donating budgets by.
+	 *
+	 * No need to use 64bit multiplication here as the first operand is
+	 * guaranteed to be smaller than WEIGHT_ONE (1<<16).
+	 *
+	 * We know that there are beneficiary nodes and the sum of the donating
+	 * hweights can't be whole; however, due to the round-ups during hweight
+	 * calculations, root_iocg->hweight_donating might still end up equal to
+	 * or greater than whole. Limit the range when calculating the divider.
 	 *
 	 * gamma = (1 - t_r') / (1 - t_r)
 	 */
 	gamma = DIV_ROUND_UP(
 		(WEIGHT_ONE - root_iocg->hweight_after_donation) * WEIGHT_ONE,
-		WEIGHT_ONE - root_iocg->hweight_donating);
+		WEIGHT_ONE - min_t(u32, root_iocg->hweight_donating, WEIGHT_ONE - 1));
 
 	/*
 	 * Calculate adjusted hwi, child_adjusted_sum and inuse for the inner