linux-kernel - Re: [SPARC32] NULL pointer derefference

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Date:	Tue, 31 Jul 2007 06:35:29 +0100 (BST)
From:	Mark Fortescue <mark@...hpc.demon.co.uk>
To:	David Miller <davem@...emloft.net>
cc:	aaw@...gle.com, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, sparclinux@...r.kernel.org,
	wli@...omorphy.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [SPARC32] NULL pointer derefference

Hi David,

>
> One possible issue is sequencing, perhaps the stack argument copy
> is occuring before the new context is setup properly on sun4c.
>

I think it is somthing related to this but too much has changed for me to 
work out what is going on. At present, I don't have a good enough 
understanding of the virtual memory system and how it interracts with the 
sun4c mmu.

The original code did a job lot of pte stuf in install_arg_page. The new 
code seems to replace this using get_user_pages but I have not worked out 
how get_user_pages gets to the point at which it allocated pte's i.e. 
maps the stack memory it is about to put the arguments into.

> Another issue might be the new flush_cache_page() call in this
> new code in fs/exec.c, there are now cases where flush_cache_page()
> will be called on kernel addresses, and sun4c's implementation might
> not like that at all.

I commented out the flush_cache_page callmade in the new code. This had no 
effect on the problem. Other tests have shown it is breaking earlier than 
this.

I am going to try to narrow down exactly where the pointer gets messed up 
as this should help.

Regards
 	Mark Fortescue.
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