blk-throttle: prevent overflow while calculating wait time

There is a problem found by code review in tg_with_in_bps_limit() that
'bps_limit * jiffy_elapsed_rnd' might overflow. Fix the problem by
calling mul_u64_u64_div_u64() instead.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220829022240.3348319-3-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index a5e495b678274fd5cb3a1b0581a9c9a8c89db48b..353bbd0886ca7a41bf9f82cd0091c0da89bd914a 100644 (file)
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -806,7 +806,7 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
                                 u64 bps_limit, unsigned long *wait)
 {
        bool rw = bio_data_dir(bio);
-       u64 bytes_allowed, extra_bytes, tmp;
+       u64 bytes_allowed, extra_bytes;
        unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
        unsigned int bio_size = throtl_bio_data_size(bio);
 
@@ -824,10 +824,8 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
                jiffy_elapsed_rnd = tg->td->throtl_slice;
 
        jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice);
-
-       tmp = bps_limit * jiffy_elapsed_rnd;
-       do_div(tmp, HZ);
-       bytes_allowed = tmp;
+       bytes_allowed = mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed_rnd,
+                                           (u64)HZ);
 
        if (tg->bytes_disp[rw] + bio_size <= bytes_allowed) {
                if (wait)