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: e08d02aa5fc9 ("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>

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index fc897bb..6e29b4d 100644 (file)
--- a/block/blk-iocost.c
+++ b/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