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

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Date:	Wed, 9 Jan 2008 02:11:00 +0100
From:	Andi Kleen <ak@...e.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>,
	Glauber de Oliveira Costa <glommer@...il.com>,
	Jan Beulich <jbeulich@...ell.com>
Subject: Re: [PATCH 00 of 10] x86: unify asm/pgtable.h

> Is _PAGE_GLOBAL causing the first if() to fall through to the second 

Yes it obviously is.


> clause?  Because otherwise it shouldn't have any effect on the pte_huge() 
> test.

>
> Gah!  This can't be right!  I think the original change_page_attr() code is 
> plain buggy.

It has a few rough edges.  I was fixing various issues in the c_p_a() patch 
series I posted recently.

The code bugs when you set the same area to the same protection several
times. That likely happens here.

The cpa series adds a helpful comment explaining this.

               /*
                 * When you're here you either set the same page to PAGE_KERNEL
                 * two times in a row or the page table reference counting is
                 * broken again. To catch the later bug for now (sorry)
                 */
                printk(KERN_ERR "address %lx\n", address);
                dump_pagetable(address);
                BUG();

I've been considering taking it out, but is there ever a good reason
to do that?  I left it in because it catches bugs in c_p_a itself.

-Andi

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