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On Feb 26, 2006, at 6:22 AM, Cherry George Mathew wrote:

> I'm not sure if you're familiar with NetBSD's uvm, but I'm trying to
> get some quick tips about this one ( kernel dmesg below ).

I'm not familiar with NetBSD's UVM, so I may need some clueing in.

> Kernel is Page Faulting with "Not Present". I suspect this is too
> early for anything really complicated, so either:
>
> - pmap_enter_vhpt() didn't do its job.

pmap_enter_vhpt() is a bit weird in that it really doesn't put the
translation in the TC. It just adds the translation to the VHPT and
takes a second trap to put it in the TC. This is really bogus as it
just adds overhead. It's simpler though.
In any case: pmap_enter_vhpt() may have done its job, but the second
trap may not have found the translation in the VHPT. I don't know
if exception.S has been taken verbatim...

> - UVM is trying to fault in a kernel page ???!!!! Isn't this what  
> pmap_growkernel is for ?

pmap_growkernel() is called by UVM if more kernel VM is needed, yes.
I presume that there's some initial amount of kernel VM and that
there's a pool of PTEs for it from which to draw. This doesn't look
like a problem because you get a "Page Not Present", not some VM
related error that it has ran out of kernel VM.

> - pmap_kenter_pa() screwed up with pte's

Dunno. Doesn't look like it.

> Also:
> This might be a really dumb question, but can you tell me why we need
> a Region 5 vhpt mapped region, when we already have a direct mapped
> Region 7 region ? Its not immediately obvious to me.

Region 5 is the region for "normal" kernel virtual memory, in the same
way that regions 0-4 are for user space virtual memory. Regions 6 & 7
are direct mapped and are mostly used to 1) provide non-cached virtual
memory references to bus space (i.e. region 6) or cached virtual memory
references to critical data-structures, such as the VHPT itself (i.e.
region 7).
Put differently region 5 is where VM managed memory comes from, while
region 6 and 7 are used by the machine dependent implementation to
support the MI functionality.


> http://cherry.zyx.in/downloads/netbsd-anvil/pmap-debug/260206.pmap.c

Looking at pmap.c, I can't see any obvious clues. The first thing to
do is figure out if the trap is normal or not. If it is, then the
problem is in handling the trap and not in having the trap itself.
Unfortunately, I can't determine that from the backtrace or pmap.c

>     trap vector = 0x14 (Page Not Present)
>     cr.iip      = 0xe00000000414d900
>     cr.ipsr     = 0x1010080a2010 (mfl,ic,dt,dfh,rt,cpl=0,it,ri=0,bn)
>     cr.isr      = 0x200000000 (code=0,vector=0,w,ei=0)
>     cr.ifa      = 0xa0000000000040b4
>     curlwp   = 0xe000000004207a00
>         pid = 0, comm =

The IFA looks normal enough though. I presume the page size is 8KB?

The advantage of using SKI is that you can set breakpoints and step
though the execution. The thing to do is, is to find out what the
memory address represents (i.e. what data structure), and how it got
put there.

I need to dig into the code more before I can be more helpful, I fear.

HTH,

BTW: I'm actually thinking that a 16KB page is slightly better, simply
because ia64 has a relatively big dataset set on average. I think Linux
also uses 16KB pages by default. Unfortunately, switching the page size
to 16KB on FreeBSD is not that easy WRT backward compatibility. You're
in a perfect position right now to switch to 16KB pages. Just a thought.

-- 
  Marcel Moolenaar         USPA: A-39004          marcel@xcllnt.net



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