linux-kernel - Re: [PATCH 00 of 10] x86: unify asm/pgtable.h

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Date:	Tue, 08 Jan 2008 17:35:26 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@...e.de>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <glommer@...il.com>,
	Jan Beulich <jbeulich@...ell.com>
Subject: Re: [PATCH 00 of 10] x86: unify asm/pgtable.h

Andi Kleen wrote:
>> of the if()?  If not, can't we just remove it and avoid this present 
>> problem?
>>     
>
> That would just hide your problem.
>   

The "problem" is a BUG() in pageattr_64.c:change_page_attr(), which to 
me looks spurious.  It arises because __PAGE_KERNEL_* doesn't contain 
_PAGE_GLOBAL, but PAGE_KERNEL_* does.  When ioremap() 
change_page_attr(), it does so in a way that guarentees that the test

	if (pgprot_val(prot) != pgprot_val(ref_prot)) {

in __change_page_attr() always succeeds.  When I folded _PAGE_GLOBAL 
into the __PAGE_KERNEL_* definitions, it mostly works except it causes 
this if() to fail, falling into the otherwise dead else clause and 
triggers a BUG().

I'm not really sure what the logic in here is supposed to be doing, but 
it seems to me that it isn't the difference between global and 
non-global mappings (I guess its really testing for cached vs uncached 
mappings).

Or to put it another way, what's the underlying rationale for making 
__PAGE_KERNEL_* not include the GLOBAL flag, but including it in the 
pgprot versions?  It means that code like this in ioremap_64.c:

		err = change_page_attr_addr(vaddr,npages,__pgprot(__PAGE_KERNEL|flags));

is subtly different from using PAGE_KERNEL | flags, because the latter 
also includes _PAGE_GLOBAL.

    J
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