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From:   Khazhy Kumykov <khazhy@chromium.org>
Date:   Wed, 4 Oct 2023 13:30:44 -0700
Message-ID: <CACGdZYLFkNs7uOuq+ftSE7oMGNbB19nm40E86xiagCFfLZ1P0w@mail.gmail.com>
Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled
To:     Tejun Heo <tj@kernel.org>
Cc:     linan666@huaweicloud.com, josef@toxicpanda.com, axboe@kernel.dk,
        yukuai3@huawei.com, cgroups@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        linan122@huawei.com, yi.zhang@huawei.com, houtao1@huawei.com,
        yangerkun@huawei.com
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On Wed, Oct 4, 2023 at 12:32=E2=80=AFPM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Thu, Sep 28, 2023 at 09:58:58AM +0800, linan666@huaweicloud.com wrote:
> > From: Li Nan <linan122@huawei.com>
> >
> > When the throttle of bps is not enabled, tg_bps_limit() returns U64_MAX=
,
> > which is be used in calculate_bytes_allowed(), and divide 0 error will
> > happen.
>
> calculate_bytes_allowed() is just
>
>   return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
>
> The only division is by HZ. How does divide by 0 happen?

We've also noticed this - haven't looked too deeply but I don't think
it's a divide by zero, but an overflow (bps_limit * jiffy_elapsed / HZ
will overflow for jiffies > HZ). mul_u64_u64_div_u64 does say it will
throw DE if the mul overflows

>
> Thanks.
>
> --
> tejun