linux-kernel - Re: [Bug Fix]: Do 32-bit table calculations in pre-processor

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Message-Id: <200907031403.34566.lkml@morethan.org>
Date:	Fri, 3 Jul 2009 14:03:17 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [Bug Fix]: Do 32-bit table calculations in pre-processor

On Fri July 3 2009, Jeremy Fitzhardinge wrote:
> [ Missed some recipients. ]
> 
> On 07/03/09 11:14, Michael S. Zick wrote:
> > Here is one I have found useful in my VIA processor bug hunting:
> >
> > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
> > index 3068388..2303d86 100644
> > --- a/arch/x86/kernel/head_32.S
> > +++ b/arch/x86/kernel/head_32.S
> > @@ -61,7 +61,7 @@
> >
> >  /* Enough space to fit pagetables for the low memory linear map */
> >  MAPPING_BEYOND_END = \
> > -       PAGE_TABLE_SIZE(((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT) << PAGE_SHIFT
> > +       PAGE_TABLE_SIZE((1<<20) - (__PAGE_OFFSET >> PAGE_SHIFT)) << PAGE_SHIFT
> >
> >   
> 
> These calculations are performed by the assembler, not the
> preprocessor.  I think this transformation looks correct (in that its an
> identity with the original), but my understanding is that the assembler
> does it calculations in arbitrary precision, so there's no need to worry
> about limiting the arithmetic to 32-bits.
> 
> >  /*
> >   * Worst-case size of the kernel mapping we need to make:
> >
> > = = =
> >
> > Before:
> >
> >   #5 [0000010000 - 0000011000]          PGTABLE ==> [0000010000 - 0000011000]
> >   #6 [0000011000 - 0000015000]          BOOTMAP ==> [0000011000 - 0000015000]
> >
> > After:
> >
> >   #5 [0000010000 - 000007d000]          PGTABLE ==> [0000010000 - 000007d000]
> >   #6 [000007d000 - 0000081000]          BOOTMAP ==> [000007d000 - 0000081000]
> >
> > Someone who knows mm check which is the reasonable value please.
> >   
> 
> 
> The PGTABLE reservation seems much too big.  I think 1 page should be
> sufficient for a system with large pages.  Even if not, 0x6d000 is way
> too large.  And they symptoms of failing to reserve the initial
> pagetable are pretty non-subtle.
> 

Random system halts, deadlocks with interrupts disabled?
Yup, that sounds familiar.

If I ever get more than a stopped machine with a glowing power light;
I will be certain to share.

Mike
>     J
> 
> 


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