linux-kernel - Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

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Date:	Wed, 18 Feb 2015 12:28:51 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

On Wed, 18 Feb 2015 20:47:48 +0100 Heinrich Schuchardt <xypron.glpk@....de> wrote:

> Hello Andrew,
> 
> thank you for your comments. Unfortunately there is no solution with
> 32-bit calculus.

How would something along the lines of

	if (PAGE_SIZE < THREAD_SIZE)
		...
	else
		...

look?

> Please, see my answers below.
> 
> As fork_init is only called once there should be not performance issue
> in using 64-bit calculus.

Sure, it's not a big issue.  But please do address the code comments
and no "temp"!
 
> I think that my patch did not cover all problems connected to max_threads.
> 
> I just had a look at the memory hotplugging code.
> Shouldn't max_threads and init_task.signal->rlim[RLIMIT_NPROC] be
> recalculated after adding or removing memory?
> This could be done in a hotplug callback.

That sounds right.  We've fixed some of these inaccuracies but there
will be many more remaining.  Searching for things like "mempages" and
"nr_free_buffer_pages" shows them up.  mem hotplug is an ongoing thing ;)

> max_threads can be set by writing to /proc/sys/kernel/threads-max.
> Shouldn't the value be checked by the same routine

Probably.

> and shouldn't
> init_task.signal->rlim[RLIMIT_NPROC] be updated?

Harder.  By this time the system has all these processes which have
inherited their rlimits from init.

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