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SubjectRe: [Bug Fix]: Do 32-bit table calculations in pre-processor
Date
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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