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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: e08d02aa5f ("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>
This commit is contained in:
Tejun Heo 2020-09-11 18:40:49 -04:00 committed by Jens Axboe
parent 0806e60f31
commit 769b628de0
1 changed files with 10 additions and 4 deletions
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14
block/blk-iocost.c
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@ -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