linux-kernel - Re: [PATCH v2] block: Use div64_ul instead of do

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Message-ID: <YaUZpZYI71K66Zka@slm.duckdns.org>
Date:   Mon, 29 Nov 2021 08:19:17 -1000
From:   Tejun Heo <tj@...nel.org>
To:     cgel.zte@...il.com
Cc:     axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        zealci@....com.cn, deng.changcheng@....com.cn
Subject: Re: [PATCH v2] block: Use div64_ul instead of do_div

On Thu, Nov 18, 2021 at 03:40:33AM +0000, cgel.zte@...il.com wrote:
> From: Changcheng Deng <deng.changcheng@....com.cn>
> 
> do_div() does a 64-by-32 division. Here the divisor is an unsigned long
> which on some platforms is 64 bit wide. So use div64_ul instead of do_div
> to avoid a possible truncation.
> 
> Reported-by: Zeal Robot <zealci@....com.cn>
> Signed-off-by: Changcheng Deng <deng.changcheng@....com.cn>
> ---
>  block/blk-throttle.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 39bb6e68a9a2..2db635d66617 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -1903,7 +1903,7 @@ static void throtl_downgrade_check(struct throtl_grp *tg)
>  
>  	if (tg->bps[READ][LIMIT_LOW]) {
>  		bps = tg->last_bytes_disp[READ] * HZ;
> -		do_div(bps, elapsed_time);
> +		bps = div64_ul(bps, elapsed_time);

Yeah, the divisor is ulong for whatever reason but the thing is in jiffies.
I have a hard time imagining it overflowing 32bit. I'm not sure how useful
this patch is.

-- 
tejun