linux-kernel - Re: Memory limits - mm_segment_t

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Message-Id: <200904301550.35112.arnd@arndb.de>
Date:	Thu, 30 Apr 2009 15:50:34 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	monstr@...str.eu
Cc:	John Williams <john.williams@...alogix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Memory limits - mm_segment_t - MAKE_MM_SEG

On Wednesday 29 April 2009, Michal Simek wrote:
> I look at some things which I need to clear for MMU Microblaze patches and
> I would like to know your opinion about.
> 
> First of all I found that almost all archs use MAKE_MM_SEG macro which 
> should be good to move to generic location (asm-generic/uaccess.h ? )
> #define MAKE_MM_SEG(s) ((mm_segment_t) { (s) })

I have a generic uaccess.h queued up, which I'm planning to submit for
2.6.30 that includes this. 

> The second thing is about place where is stored limit for processes -> mm_segment_t structure
> 
> Where is the proper location for storing mm_segment_t? Some arch use
> thread_info some of them thread_struct

The method that is used here is different on some architectures.
Most of them use the address limit, which is a property of the
thread, and sensibly belongs into the thread_info. s390 and possibly
others have separate address spaces for user access and use a CPU
feature for this, which belongs into thread_struct.

> Here is the small table for cpus which are in linux kernel and location and type for them.
> The most of them uses thread_info structure for it and name is addr_limit.
> The location of mm_segment_t is different too -> we should move it to any generic location too.
> What do you think?

Sounds fair to me. I think uaccess.h is the best place for this, or maybe
a new mm_segment.h, but not segment.h which is used traditionally for
real segments on x86.

	Arnd <><
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