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Date: Tue, 3 Apr 2012 09:56:06 -0500
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From: Mark Tinguely <marktinguely@gmail.com>
To: vasanth rao naik sabavat <vasanth.raonaik@gmail.com>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: question about amd64 pagetable page allocation.
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On Tue, Apr 3, 2012 at 8:33 AM, vasanth rao naik sabavat
<vasanth.raonaik@gmail.com> wrote:
> Hi,
>
> I am trying to understand the page table page allocation for a process in
> FBSD6.1. I see that the page table pages are allocated by vm_page_alloc()=
.
> I believe the virtual address for this allocated page can be derived by
> PHYS_TO_DMAP(VM_PAGE_TO_PHYS(m)), however when I compare this address wit=
h
> the virtual address accessed in pmap_remove_pages() they are not the same=
.
>
> The virtual address of a *PTE in pmap_remove_pages() is something like
> *0xffff800000643200
> *
> However, the virtual address of the page allocated by vm_page_alloc() is =
=A0*
> 0xffffff033c6a0000
>
> *I would also like to understand the importance of loc_PTmap, I believe t=
he
> pmap_remove_pages() is derving the page table page addresses from loc_PTm=
ap?
> (kgdb) p loc_PTmap
> Cannot access memory at address 0xffff800000000000
>
> How do we relate the loc_PTmap to the page table pages allocated by
> vm_page_alloc() in _pmap_allocpte().
>
> Thanks,
> Vasanth



The answer to your questions will be more obvious when you get an
understanding of the Recursive Page Tables.

--Mark Tinguely.