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Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled
To:     Khazhy Kumykov <khazhy@chromium.org>,
        Oleg Nesterov <oleg@redhat.com>
Cc:     Yu Kuai <yukuai1@huaweicloud.com>,
        Li Nan <linan666@huaweicloud.com>, 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>
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From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <e9165cd0-9c9d-1d1a-1c5b-402556a1a31f@huaweicloud.com>
Date:   Mon, 16 Oct 2023 09:46:59 +0800
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Hi,

在 2023/10/14 5:51, Khazhy Kumykov 写道:
> Looking at the generic mul_u64_u64_div_u64 impl, it doesn't handle
> overflow of the final result either, as far as I can tell. So while on
> x86 we get a DE, on non-x86 we just get the wrong result.
> 
> (Aside: after 8d6bbaada2e0 ("blk-throttle: prevent overflow while
> calculating wait time"), setting a very-high bps_limit would probably
> also cause this crash, no?)
> 
> Would it be possible to have a "check_mul_u64_u64_div_u64_overflow()",
> where if the result doesn't fit in u64, we indicate (and let the
> caller choose what to do? Here we should just return U64_MAX)?
> 
> Absent that, maybe we can take inspiration from the generic
> mul_u64_u64_div_u64? (Forgive the paste)
> 
>   static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long jiffy_elapsed)
>   {
> +       /* Final result probably won't fit in u64 */
> +       if (ilog2(bps_limit) + ilog2(jiffy_elapsed) - ilog2(HZ) > 62)

I'm not sure, but this condition looks necessary, but doesn't look
sufficient, for example, jiffy_elapsed cound be greater than HZ, while
ilog2(jiffy_elapsed) is equal to ilog2(HZ).

Thanks,
Kuai

> +               return U64_MAX;
>          return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
>   }
> 
> .
>