The page directory is also pointed at by CR3, and that pointer is a Physical
Address not a Virtual Address, so it would be a Real Bad Idea™ for the
root of the page descriptors (i.e. the Page Directory) to not be resident.
=====================
Mark Roddy
Hollis Technology Solutions
www.hollistech.com
xxxxx@hollistech.com
-----Original Message-----
From: Nick Ryan [mailto:xxxxx@nryan.com]
Sent: Thursday, July 24, 2003 9:03 PM
To: Windows System Software Developers Interest List
Subject: [ntdev] RE: FOR FAQ: Re: Context switching … (now double fault)
The page directory has entries that map back to itself (possible because
the format of page table entries and page directory entries are
compatible). There are special bits in the PDE/PTE that mark a page as
being backed by a page file. When Windows builds up the entries for the
page directory, I assume it doesn’t set these bits, indicating that the
page directory itself cannot be paged out.
Prokash Sinha wrote:
I think I’m not clear enough. But for sure, why would anyone have a
backup file for nonpaged pool, no need ???I’m asking this to clarify myself, what is the reason that
a paging directory can not be pageable. There must be a reason (either
the h/w with the handler would not handle it, due to, who knows,
nested page faults?? or just perf reason !!!thanx
prokash-----Original Message-----
From: xxxxx@lists.osr.com
[mailto:xxxxx@lists.osr.com]On Behalf Of Nick Ryan
Sent: Thursday, July 24, 2003 5:30 PM
To: Windows System Software Developers Interest List
Subject: [ntdev] RE: FOR FAQ: Re: Context switching … (now double
fault)The page directories cannot be paged out. Not all virtual memory pages
in the system are backed by the page file - NonPaged pool, for
instance. The scenario you are describing is simply not possible.Prokash Sinha wrote:
>Yes I am rusty on this, and still not clear ( about the page
>directories, and tables). Given a logical(linear) addr, h/w MMU unit
>would raise a flag if the pagetable containing
>directory info is not resident !!!, pagefault handler would field this
>fault!!, bring the
>page from the system page file ( no file specific mem map, it is the page
>file we often
>modify the size of for different options of krnl dump ??? ), now while the
>page is fetched, it
>is a directory, so we need to get the pagetable, then another fault from
>pagetable to actual page
>(assuming one lvl of indirection) …
>
>By your logic it seems like this could be handled, may be just the
>performance reason to have the directory and resident all the time!!.
>
>If pagetable is pageable, (w/one level of inderection), we may haveanother
>actual page to bring in
>so then it answers my question…
>
>thanx
>prokash
>
>-----Original Message-----
>From: xxxxx@lists.osr.com
>[mailto:xxxxx@lists.osr.com]On Behalf Of Nick Ryan
>Sent: Thursday, July 24, 2003 3:49 PM
>To: Windows System Software Developers Interest List
>Subject: [ntdev] RE: FOR FAQ: Re: Context switching … (now double
>fault)
>
>
>That type of ‘double-fault’ can be handled, depending on the
>situation. If the page fault occurs on a memory-mapped file and Mm
>calls back into the FSD, and the FSD then faults again accessing paged
>memory backed by the swap file, this second fault is perfectly legal.
>For obvious reasons, of course, you can’t fault on the same entity
>that you are already handling a fault on (which is why an FSD or
>filter can’t touched paged pool on the paging file I/O path).
>
>Prokash Sinha wrote:
>
>
>
>>Thanx again.
>>
>>But is it possible to have a pagefault, while we are in a pagefault ?
>>
>>-prokash
>>
>>-----Original Message-----
>>From: xxxxx@lists.osr.com
>>[mailto:xxxxx@lists.osr.com]On Behalf Of Nick Ryan
>>Sent: Thursday, July 24, 2003 3:04 PM
>>To: Windows System Software Developers Interest List
>>Subject: [ntdev] FOR FAQ: Re: Context switching … (now double
>>fault)
>>
>>
>>A double fault occurs as a result of a driver (usually a filesystem
>>filter driver or filesystem driver) overflowing the kernel stack. You
>>will see bug check code 0x7F (UNEXPECTED_KERNEL_MODE_TRAP), with
>>first parameter as 0x00000008. From the DDK:
>>
>>“0x00000008, or Double Fault, is when an exception occurs while
>>trying to call the handler for a prior exception. Normally, the two
>>exceptions can be handled serially. However, there are several
>>exceptions that cannot be handled serially, and in this situation the
>>processor signals a double fault. There are two common causes of a
>>double fault:
>>
>>1. A kernel stack overflow. This occurs when a guard page is hit, and
>>then the kernel tries to push a trap frame. Since there is no stack
>>left, a stack overflow results, causing the double fault. If you
>>suspect this has occurred, use the !thread debugger extension to
>>determine the stack limits, and then use the KB (Display Stack
>>Backtrace) debugger command with a large parameter (for example, kb
>>100) to display the full stack.
>>
>>2. A hardware problem.”
>>
>>Prokash Sinha wrote:
>>
>>
>>
>>
>>>Nick,
>>>
>>>Is it the reason that a double fault would occur, if the directories
>>>are paged out !!!
>>>
>>>-prokash
>>>
>>>-----Original Message-----
>>>From: xxxxx@lists.osr.com
>>>[mailto:xxxxx@lists.osr.com]On Behalf Of Nick Ryan
>>>Sent: Thursday, July 24, 2003 1:33 PM
>>>To: Windows System Software Developers Interest List
>>>Subject: [ntdev] Re: Context switching …
>>>
>>>
>>>Page tables can be paged out, but page directories cannot be.
>>>
>>>James Antognini wrote:
>>>
>>>
>>>
>>>
>>>
>>>>I believe page tables are themselves pageable. That suggests very
>
>strongly
>
>
>>>that
>>>
>>>
>>>
>>>
>>>>if their storage is stolen and they are paged back in, they will
>>>>assumea
>>>>possibly different storage location.
>>>>
>>>>Nick Ryan wrote:
>>>>
>>>>
>>>>
>>>>>From what I understand, all page table directories for all
>>>>>processes
>>>>
>>>>
>>>>
>>>>>exist at different kernel-mode addresses simultaneously.
>>>>
>>>>
>>>>–
>>>>If replying by e-mail, please remove “nospam.” from the address.
>>>>
>>>>James Antognini
>>>>Windows DDK MVP
>>>>
>>>>
>>>>
>>>>—
>>>>You are currently subscribed to ntdev as: xxxxx@nryan.com To
>>>>unsubscribe send a blank email to xxxxx@lists.osr.com
>>>>
>>>
>>>
>>>–
>>>- Nick Ryan (MVP for DDK)
>>>
>>>
>>>
>>>—
>>>You are currently subscribed to ntdev as: xxxxx@vormetric.com To
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>>>
>>>
>>>
>>>—
>>>You are currently subscribed to ntdev as: xxxxx@nryan.com To
>>>unsubscribe send a blank email to xxxxx@lists.osr.com
>>>
>>
>>
>>–
>>- Nick Ryan (MVP for DDK)
>>
>>
>>
>>—
>>You are currently subscribed to ntdev as: xxxxx@vormetric.com To
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>>
>>
>>
>>—
>>You are currently subscribed to ntdev as: xxxxx@nryan.com
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>
>
>–
>- Nick Ryan (MVP for DDK)
>
>
>
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>
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>
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>—
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