Commit 769b628d authored by Tejun Heo's avatar Tejun Heo Committed by Jens Axboe
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 0806e60f

block/blk-iocost.c

+10 −4
Original line number Diff line number Diff line
@@ -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

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