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Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled
To:     Khazhy Kumykov <khazhy@chromium.org>,
        Yu Kuai <yukuai1@huaweicloud.com>
Cc:     Oleg Nesterov <oleg@redhat.com>, Li Nan <linan666@huaweicloud.com>,
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        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: <bdba4f58-1adf-32ab-5f5a-0fbb1a2ae6c2@huaweicloud.com>
Date:   Tue, 17 Oct 2023 10:38:25 +0800
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Hi,

在 2023/10/17 4:06, Khazhy Kumykov 写道:
> On Sun, Oct 15, 2023 at 6:47 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>
>> 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).
> I believe 62 is correct, although admittedly it's less "intuitive"
> than the check in mul_u64_u64_div_u64()....
> 
> The result overflows if log2(A * B / C) >= 64, so we want to ensure that:
> log2(A) + log2(B) - log2(C) < 64
> 
> Given that:
> ilog2(A) <= log2(A) < ilog2(A) + 1  // truncation defn
> It follows that:
> -log2(A) <= -ilog2(A)  // Inverse rule
> log2(A) - 1 < ilog2(A)
> 
> Starting from:
> ilog2(A) + ilog2(B) - ilog2(C) <= X
> 
> We can show:
> (log2(A) - 1) + (log2(B) - 1) + (-log2(C)) < ilog2(A) + ilog2(B) +
> (-ilog2(C)) // strict inequality here since the substitutions for A
> and B terms are strictly less
> (log2(A) - 1) + (log2(B) - 1) + (-log2(C)) < X
> log2(A) + log2(B) - log2(C) < X + 2
> 
> So for X = 62, log2(A) + log2(B) - log2(C) < 64 must be true, and we
> must be safe from overflow.
> 
> So... by converse, if ilog2(A) + ilog2(B) - ilog2(C) > 62, we cannot
> guarantee that the result will not overflow - thus we bail out.
> 
> // end math

Thanks for the explanation, I understand that, so the problem is that
if the above condition(>62) match, the result may not overflow, but
U64_MAX is returned here, this is not correct but I guess this doesn't
matter in real word, it's impossible that user will issue more than
1<<63 bytes IO in an extended slice.

I'm good with this fix with some comments.

Thanks,
Kuai

> 
> It /is/ less exact than your proposal (sufficient, but not necessary),
> but saves an extra 128bit mul.
> 
> I mostly just want us to pick /something/, since 6.6-rc and the LTSs
> with the patch in question are busted currently. :)
> 
> 
> 
>>
>> Thanks,
>> Kuai
>>
>>> +               return U64_MAX;
>>>           return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
>>>    }
>>>
>>> .
>>>
>>
> 
> .
>