linux-kernel - Re: [PATCH 2/4 v6] kernel/fork.c: avoid division by zero

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Date:	Mon, 16 Mar 2015 08:41:46 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Davidlohr Bueso <dave@...olabs.net>,
	David Rientjes <rientjes@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Fabian Frederick <fabf@...net.be>,
	Guenter Roeck <linux@...ck-us.net>,
	"H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...com>,
	Joe Perches <joe@...ches.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Jonathan Corbet <corbet@....net>,
	Kees Cook <keescook@...omium.org>,
	Michael Marineau <mike@...ineau.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/4 v6] kernel/fork.c: avoid division by zero


* Heinrich Schuchardt <xypron.glpk@....de> wrote:

> PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
> THREAD_SIZE.
> 
> E.g. architecture hexagon may have page size 1M and thread size 4096.
> This would lead to a division by zero in the calculation of max_threads.
> 
> With 32-bit calculation there is no solution which delivers valid results
> for all possible combinations of the parameters.
> The code is only called once.
> Hence a 64-bit calculation can be used as solution.
> 
> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> ---
>  kernel/fork.c | 35 ++++++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bf1ff00..69ff08f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -88,6 +88,16 @@
>  #include <trace/events/task.h>
>  
>  /*
> + * Minimum number of threads to boot the kernel
> + */
> +#define MIN_THREADS 20
> +
> +/*
> + * Maximum number of threads
> + */
> +#define MAX_THREADS FUTEX_TID_MASK
> +
> +/*
>   * Protected counters by write_lock_irq(&tasklist_lock)
>   */
>  unsigned long total_forks;	/* Handle normal Linux uptimes. */
> @@ -258,18 +268,25 @@ void __init __weak arch_task_cache_init(void) { }
>   */
>  static void set_max_threads(void)
>  {
> -	/*
> -	 * The default maximum number of threads is set to a safe
> -	 * value: the thread structures can take up at most half
> -	 * of memory.
> -	 */
> -	max_threads = totalram_pages / (8 * THREAD_SIZE / PAGE_SIZE);
> +	u64 threads;
>  
>  	/*
> -	 * we need to allow at least 20 threads to boot a system
> +	 * The number of threads shall be limited such that the thread
> +	 * structures may only consume a small part of the available memory.
>  	 */
> -	if (max_threads < 20)
> -		max_threads = 20;
> +	if (fls64(totalram_pages) + fls64(PAGE_SIZE) > 64)
> +		threads = MAX_THREADS;
> +	else
> +		threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
> +				    (u64) THREAD_SIZE * 8UL);
> +
> +	if (threads > MAX_THREADS)
> +		threads = MAX_THREADS;
> +
> +	if (threads < MIN_THREADS)
> +		threads = MIN_THREADS;
> +
> +	max_threads = (int) threads;
>  }

So why does this patch do two things:

	- parametrizes set_max_threads() via defines
	- fixes a bug

?

Those two things should be done in two separate patches, first the 
introduction of parameters, then the fixing of the bug.

I suggested this in my first review: separate out and keep the fix 
portion of the series minimal.

Thanks,

	Ingo
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