linux-kernel - Re: [PATCH] blk-throttle: Calculate allowed value only when the throttle is enabled

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Date:   Sat, 7 Oct 2023 17:16:08 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Li Nan <linan666@...weicloud.com>,
        Khazhy Kumykov <khazhy@...omium.org>, tj@...nel.org,
        josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, houtao1@...wei.com, yangerkun@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled

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.