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Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled
To:     Oleg Nesterov <oleg@redhat.com>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Li Nan <linan666@huaweicloud.com>,
        Khazhy Kumykov <khazhy@chromium.org>, tj@kernel.org,
        josef@toxicpanda.com, axboe@kernel.dk, cgroups@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, houtao1@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230928015858.1809934-1-linan666@huaweicloud.com>
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 <4ace01e8-6815-29d0-70ce-4632818ca701@huaweicloud.com>
 <20231005162417.GA32420@redhat.com>
 <0a8f34aa-ced9-e613-3e5f-b5e53a3ef3d9@huaweicloud.com>
 <20231007151607.GA24726@redhat.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <21843836-7265-f903-a7d5-e77b07dd5a71@huaweicloud.com>
Date:   Sun, 8 Oct 2023 09:26:25 +0800
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Hi,

?? 2023/10/07 23:16, Oleg Nesterov д??:
> On 10/07, Yu Kuai wrote:
>>
>>>>> probably need to remove the mul_u64_u64_div_u64 and check for
>>>>> overflow/potential overflow ourselves?
>>>
>>> probably yes...
>>
>> How about this?
>>
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index 1101fb6f6cc8..5482c316a103 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -723,6 +723,10 @@ static unsigned int calculate_io_allowed(u32
>> iops_limit,
>>
>>   static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long
>> jiffy_elapsed)
>>   {
>> +       if (jiffy_elapsed > HZ &&
>> +           bps_limit > mul_u64_u64_div_u64(U64_MAX, (u64)HZ,
>> (u64)jiffy_elapsed);
>> +               return U64_MAX;
>> +
> 
> I can't suggest anything better...
> 
> but I do not know if it is possible that HZ > jiffy_elapsed. If yes, then
> mul_u64_u64_div_u64() above is not safe too.

Well, 'jiffy_elapsed > HZ' is judged before mul_u64_u64_div_u64().

Overflow can only happen if the above 2 conditions passed, and U64_MAX
is returned.

Thanks,
Kuai

> 
> Oleg.
> 
> .
>