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

[Khazhy Kumykov]

On Wed, Oct 4, 2023 at 12:32 PM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Thu, Sep 28, 2023 at 09:58:58AM +0800, linan666@xxxxxxxxxxxxxxx wrote:
> > From: Li Nan <linan122@xxxxxxxxxx>
> >
> > 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