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Date:   Sat, 7 Oct 2023 17:16:08 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     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>
Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled
Message-ID: <20231007151607.GA24726@redhat.com>
References: <20230928015858.1809934-1-linan666@huaweicloud.com>
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 <4ace01e8-6815-29d0-70ce-4632818ca701@huaweicloud.com>
 <20231005162417.GA32420@redhat.com>
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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.

Oleg.