linux-kernel - Re: [PATCH] VMI: remove CONFIG_DEBUG_PAGE

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Date:	Fri, 06 Jul 2007 13:13:48 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Chris Wright <chrisw@...s-sol.org>
CC:	Stefan Richter <stefanr@...6.in-berlin.de>,
	"Robert P. J. Day" <rpjday@...dspring.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.osdl.org
Subject: Re: [PATCH] VMI: remove CONFIG_DEBUG_PAGE_TYPE and associated	bitrotted
 code

Chris Wright wrote:
> * Stefan Richter (stefanr@...6.in-berlin.de) wrote:
>   
>>> -#ifdef CONFIG_DEBUG_PAGE_TYPE
>>> +#if 0 /* debug page type */
>>>       
> <snip>
>   
>> This misnamed CONFIG_DEBUG_PAGE_TYPE (it's not a Kconfig variable) has
>> about 120 lines debug code dangling on it.  So, replacing it by #if 0
>> will hopefully motivate a kind janitor to send a removal patch for that
>> debug code eventually.  I don't do so just now because that code went in
>> between 2.6.20 and 2.6.21-rc1, i.e. not so long ago.
>>     
>
> This is Zach's code, his final call.  I know it was pretty useful early
> on, and used to be an actual Kconfig option for VMI.  However, it's
> completely disconnected; the setup call to vmi_apply_boot_page_allocations
> isn't merged and the page->type field isn't either (no surprise on that),
> and some of the VMI_PAGE_ constants have changed names.  Clearly, it is
> ripe for bitrot (already has !CONFIG_NEED_MULTIPLE_NODES dependency,
> dunno if VMI has the same limitation).  It definitely should not have
> a misleading Kconfig name.  I'd nuke it all rather than #if 0.
>
> thanks,
> -chris
>   

I'd rather keep it, even with bitrot - it was non-trivial to get 
correct, and found many surprises in the code; most notably, it can detect

1) PTE writes to pages not declared as page tables
2) Failure to allocate or de-allocate page tables using the paravirt-ops API
3) PTE writes using the wrong level operations

These are most useful properties; in fact, I would like to extend the 
code for 64-bit paravirt-ops and 4-level paging, so rather not kill it 
until then.

I never merged the whole bit upstream because it added a field to struct 
page.

Zach
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