linux-kernel - RE: [PATCH] mm/page_alloc.c: Use div64_ul instead of do

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Date:   Fri, 7 Jan 2022 09:15:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'cgel.zte@...il.com'" <cgel.zte@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Zeal Robot <zealci@....com.cn>
Subject: RE: [PATCH] mm/page_alloc.c: Use div64_ul instead of do_div

From: cgel.zte@...il.com
> Sent: 07 January 2022 02:18
> 
> 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.

You really need to check the domain of the divisor, not its type.
Just because the type is 64 bits doesn't mean that the value can be
anywhere near even 32 bits.

	David

> 
> Reported-by: Zeal Robot <zealci@....com.cn>
> Signed-off-by: Changcheng Deng <deng.changcheng@....com.cn>
> ---
>  mm/page_alloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8dd6399bafb5..60469b616ac1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8431,7 +8431,7 @@ static void __setup_per_zone_wmarks(void)
> 
>  		spin_lock_irqsave(&zone->lock, flags);
>  		tmp = (u64)pages_min * zone_managed_pages(zone);
> -		do_div(tmp, lowmem_pages);
> +		tmp = div64_ul(tmp, lowmem_pages);
>  		if (is_highmem(zone)) {
>  			/*
>  			 * __GFP_HIGH and PF_MEMALLOC allocations usually don't
> @@ -8804,7 +8804,7 @@ void *__init alloc_large_system_hash(const char *tablename,
>  	/* limit allocation size to 1/16 total memory by default */
>  	if (max == 0) {
>  		max = ((unsigned long long)nr_all_pages << PAGE_SHIFT) >> 4;
> -		do_div(max, bucketsize);
> +		max = div64_ul(max, bucketsize);
>  	}
>  	max = min(max, 0x80000000ULL);
> 
> --
> 2.25.1

-
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