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

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Date:   Thu, 5 Oct 2023 10:15:06 -0700
From:   Khazhy Kumykov <khazhy@...omium.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Li Nan <linan666@...weicloud.com>, tj@...nel.org,
        josef@...icpanda.com, axboe@...nel.dk, yukuai3@...wei.com,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        houtao1@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled

On Thu, Oct 5, 2023 at 10:05 AM Oleg Nesterov <oleg@...hat.com> wrote:
>
> sorry, didn't notice this part before.
>
> I am not a asm expert (to say at least;) but
>
> On 10/05, Li Nan wrote:
> >
> > When (a * mul) overflows, a divide 0 error occurs in
> > mul_u64_u64_div_u64().
>
> Just in case... No, iirc it is divq which triggers #DE when the
> result of division doesn't fit u64.
Yeah, sorry for my incorrect wording here - but we're probably seeing
exactly that the final result doesn't fit in u64. (I wasn't familiar
with the intermediary registers here, thanks for explaining)
>
> (a * mul) can't overflow, the result is 128-bit rax:rdx number.
>
> Oleg.
>