linux-kernel - Re: Oops mystery

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Message-ID: <51E0348A.2030208@opengridcomputing.com>
Date:	Fri, 12 Jul 2013 11:53:30 -0500
From:	Steve Wise <swise@...ngridcomputing.com>
To:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Oops mystery

On 7/12/2013 11:48 AM, Dave Jones wrote:
> On Fri, Jul 12, 2013 at 10:48:21AM -0500, Steve Wise wrote:
>   
>   > So 'movb   $0x0,0xe(%rax,%rdx,1)' should be storing 0 into the byte
>   > location:
>   >
>   > %rax + 0xe + (%rdx * 1) ==
>   > 0x40fc+ 0xe + 0xffff8808b5500000 ==
>   > 0xffff8808b5540fce.
>   >
>   > That address is readable in the crash dump:
>   >
>   > crash> x/8b 0x0000000000040fc0+0xe+0xffff8808b5500000
>   > 0xffff8808b5540fce:     0x00    0x00    0x00    0x00    0x00 0x00
>   > 0x00    0x00
>   >
>   > And why does the page fault show 0x40fc0 as the faulting address?  It
>   > should be 0xffff8808b5540fce and it shouldn't have caused a page fault.
>   >
>   > What am I missing?
>
> Random guess: Is that page marked read-only perhaps ?

It shouldn't be. :)  How can I get this info via the crash dump? The 
memory was allocated with dma_alloc_coherent().   Why would the page 
fault occur on 0x40fc0 though?  That makes me think my analysis so far 
is incorrect.


Steve.
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