linux-kernel - Re: unify pagetable accessors patch causes double fault II

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Message-ID: <20080114222349.GA23409@one.firstfloor.org>
Date:	Mon, 14 Jan 2008 23:23:49 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: unify pagetable accessors patch causes double fault II

> OK, I see the problem.  The problem is that the _PAGE_X defines are 
> defined with _AC(UL, 1 << _PAGE_BIT_X), which has unsigned long type.  
> This means that ~_PAGE_X also has unsigned long type, and so when cast 
> to 64-bit in pte_mkX, it ends up &ing the pte with 0x00000000ffffffxxx, 
> with predictable results.

Actually I fixed some of that -- see the pgtable-nx patch on firstfloor -- but
it still doesn't work. Or maybe my patch was not complete.
> 
> The original code just used signed constants for the _PAGE_X 
> definitions, which will sign-extend when cast to 64-bit, and so have the 
> upper bits set when masking.  (Well, actually, the old code just 
> operated on pte_low, so the problem didn't arise; however, pgtable_64.h 
> also uses integers for its _PAGE_X, which has the same sign-extended 
> 32->64 casting property).
> 
> I'll put together a fixup patch now.

I'm leaving now but can test later.

-Andi
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