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Subject: Re: [PATCH v2] blk-iocost: fix shift-out-of-bounds in
 iocg_hick_delay()
To:     Tejun Heo <tj@kernel.org>, Li Jinlin <lijinlin3@huawei.com>
Cc:     josef@toxicpanda.com, axboe@kernel.dk, cgroups@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        liuzhiqiang26@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20221128030413.882998-1-lijinlin3@huawei.com>
 <Y4US2vFmR4pnw08Z@slm.duckdns.org>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <bbc5e21f-9e77-41bb-5763-36bd905b52a0@huaweicloud.com>
Date:   Tue, 29 Nov 2022 09:14:58 +0800
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Hi,

?? 2022/11/29 3:58, Tejun Heo д??:
> On Mon, Nov 28, 2022 at 11:04:13AM +0800, Li Jinlin wrote:
>>   	/* calculate the current delay in effect - 1/2 every second */
>>   	tdelta = now->now - iocg->delay_at;
>>   	if (iocg->delay)
>> -		delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
>> +		delay = iocg->delay >>
>> +			min_t(u64, div64_u64(tdelta, USEC_PER_SEC), 63);
> 
> I replied earlier but the right thing to do here is setting delay to 0 if
> the shift is >= 64.

Perhaps following change will make more sense?

@@ -1322,18 +1323,19 @@ static bool iocg_kick_delay(struct ioc_gq *iocg, 
struct ioc_now *now)
  {
         struct ioc *ioc = iocg->ioc;
         struct blkcg_gq *blkg = iocg_to_blkg(iocg);
-       u64 tdelta, delay, new_delay;
+       u64 delay = 0;
+       u64 new_delay;
         s64 vover, vover_pct;
         u32 hwa;

         lockdep_assert_held(&iocg->waitq.lock);

         /* calculate the current delay in effect - 1/2 every second */
-       tdelta = now->now - iocg->delay_at;
-       if (iocg->delay)
+       if (iocg->delay && now->now > iocg->delay_at) {
+               u64 tdelta = now->now - iocg->delay_at;
+
                 delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
-       else
-               delay = 0;
+       }

> 
> Thanks.
>