linux-kernel - Re: [PATCH v3 2/2] blk-throttle: kill the no longer needed overflow check in calculate_bytes

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Message-ID: <FD649A73-0E0B-4518-9A27-B65DD583C99F@zytor.com>
Date: Sun, 17 Aug 2025 10:06:38 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: David Laight <david.laight.linux@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
CC: Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Li RongQing <lirongqing@...du.com>, Yu Kuai <yukuai3@...wei.com>,
        Khazhismel Kumykov <khazhy@...omium.org>, Jens Axboe <axboe@...nel.dk>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v3 2/2] blk-throttle: kill the no longer needed overflow check in calculate_bytes_allowed()

On August 17, 2025 5:50:13 AM PDT, David Laight <david.laight.linux@...il.com> wrote:
>On Fri, 15 Aug 2025 18:41:02 +0200
>Oleg Nesterov <oleg@...hat.com> wrote:
>
>> Now that mul_u64_u64_div_u64() can't crash there is no need to check for
>> possible overflow in calculate_bytes_allowed().
>> 
>> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>> ---
>>  block/blk-throttle.c | 6 ------
>>  1 file changed, 6 deletions(-)
>> 
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index 397b6a410f9e..66339e22cc85 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -601,12 +601,6 @@ static unsigned int calculate_io_allowed(u32 iops_limit,
>>  
>>  static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long jiffy_elapsed)
>>  {
>> -	/*
>> -	 * Can result be wider than 64 bits?
>> -	 * We check against 62, not 64, due to ilog2 truncation.
>> -	 */
>> -	if (ilog2(bps_limit) + ilog2(jiffy_elapsed) - ilog2(HZ) > 62)
>> -		return U64_MAX;
>>  	return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
>
>Not directly related, but the two (u64) casts are pointless and can be removed.
>
>	David
>
>>  }
>>  
>

It's also rather broken, because a division with a constant can be implemented as a multiply, and both gcc and clang knows how to do that.