puzzle about about page fault in sigma0 space

Bob <[email protected]> Wed, 10 Sep 2008 10:30:13 +0800 (CST)

Newsgroups	gmane.comp.micro-kernel.l4.l4ka.general
Message-ID	<[email protected]>

Hi, all 
        I am sorry for trouble you again. I am puzzled about page fault in sigma0 space.
       My puzzle is that when access a virt address below 256M in sigma0 space, and a page fault will occur.
Then the page fault process will  make phys : virt 1 : 1 mapping through map_sigma0() funtion.
       Then it can access the phys address which is below 256M ?  But these low 256M addresses is kernel used. 
And shouldn't be accessed by user thread directly. Or else the kernel data will be distroyed .
       How to explain this?

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Best regards
Bob