linux-kernel - Re: [PATCH] fork_init: fix division by zero

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Message-ID: <Pine.LNX.4.64.0812100944080.25126@vixen.sonytel.be>
Date:	Wed, 10 Dec 2008 09:44:46 +0100 (CET)
From:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>
To:	Yuri Tikhonov <yur@...raft.com>
cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Detlev Zundel <dzu@...x.de>, Wolfgang Denk <wd@...x.de>,
	Milton Miller <miltonm@....com>, Ilya Yanok <yanok@...raft.com>
Subject: Re: [PATCH] fork_init: fix division by zero

On Tue, 9 Dec 2008, Yuri Tikhonov wrote:
> The following patch fixes divide-by-zero error for the
> cases of really big PAGE_SIZEs (e.g. 256KB on ppc44x).
> Support for such big page sizes on 44x is not present in the
> current kernel yet, but coming soon.
> 
> Also this patch fixes the comment for the max_threads
> settings, as this didn't match the things actually done
> in the code.
> 
> Signed-off-by: Yuri Tikhonov <yur@...raft.com>
> Signed-off-by: Ilya Yanok <yanok@...raft.com>
> ---
>  kernel/fork.c |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2a372a0..b0ac2fb 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -181,10 +181,14 @@ void __init fork_init(unsigned long mempages)
>  
>  	/*
>  	 * The default maximum number of threads is set to a safe
> -	 * value: the thread structures can take up at most half
> -	 * of memory.
> +	 * value: the thread structures can take up at most
> +	 * (1/8) part of memory.
>  	 */
> +#if (8 * THREAD_SIZE) > PAGE_SIZE
>  	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> +#else
> +	max_threads = mempages * PAGE_SIZE / (8 * THREAD_SIZE);
                      ^^^^^^^^^^^^^^^^^^^^
> +#endif

Can't this overflow, e.g. on 32-bit machines with HIGHMEM?

With kind regards,

Geert Uytterhoeven
Software Architect

Sony Techsoft Centre Europe
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