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From: John Baldwin <jhb@freebsd.org>
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	(bigwig.baldwin.cx); Mon, 20 Aug 2012 08:31:37 -0400 (EDT)
Cc: Monthadar Al Jaberi <monthadar@gmail.com>
Subject: Re: kernel page fult for a valid pointer?
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On Saturday, August 18, 2012 2:29:26 pm Monthadar Al Jaberi wrote:
> Hi,
> 
> I am wondering is there a reason for getting "Fatal trap 12: page
> fault while in kernel mode" "supervisor read, page not present" for an
> address used to be valid in kernel space?
> 
> I dont really understand why I am getting this, I added a hardware
> watchpoint on the address, and when I got to the debuger I could read
> the memory content and dump for that address. But when I continue from
> the debugger I get the panic and now when I try to read the memory
> content I get *** error reading from address ce733000 ***.

Whatever memory was there might have been unmapped?  For example,
memory pointed to by I/O buffers (struct buf/bio) use transient
mappings that are only valid while an I/O request is in progress.

-- 
John Baldwin