linux-kernel - Re: [PATCH 1/3 v3] kernel/fork.c: avoid division by zero

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Date:	Sun, 22 Feb 2015 08:58:25 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kees Cook <keescook@...omium.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andy Lutomirski <luto@...capital.net>,
	Michael Marineau <mike@...ineau.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jens Axboe <axboe@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Guenter Roeck <linux@...ck-us.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3 v3] 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 calculus 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.
> 
> The calculation of max_threads is moved to a separate function.
> This allows future patches to use the same logic, e.g. when
>  - max_threads is set by writing to /proc/sys/kernel/threads-max
>  - when adding or removing memory.
> 
> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> ---
>  kernel/fork.c | 59 ++++++++++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 42 insertions(+), 17 deletions(-)

So it would be nice to split this patch into two parts: 
first do a patch that splits out a version of 
set_max_threads() that matches current behavior (including 
the bug).

Then a second patch that fixes the bug.

This would make it much easier to review.

Thanks,

	Ingo
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