Re: [PATCH 2.6.14] mm: 8xx MM fix for

On Mon, Nov 07, 2005 at 08:16:18AM -0200, Marcelo Tosatti wrote:

Joakim!

On Mon, Nov 07, 2005 at 03:32:52PM +0100, Joakim Tjernlund wrote:

quoted

Hi Marcelo

[SNIP] 

quoted

The root of the problem are the changes against the 8xx TLB 
handlers introduced
during v2.6. What happens is the TLBMiss handlers load the 
zeroed pte into
the TLB, causing the TLBError handler to be invoked (thats 
two TLB faults per 
pagefault), which then jumps to the generic MM code to setup the pte.

The bug is that the zeroed TLB is not invalidated (the same reason
for the "dcbst" misbehaviour), resulting in infinite TLBError faults.

Dan, I wonder why we just don't go back to v2.4 behaviour.

This is one reason why it is the way it is:
http://ozlabs.org/pipermail/linuxppc-embedded/2005-January/016382.html
This details are little fuzzy ATM, but I think the reason for the
current
impl. was only that it was less intrusive to impl.

Ah, I see. I wonder if the bug is processor specific: we don't have such
changes in our v2.4 tree and never experienced such problem.

It should be pretty easy to hit it right? (instruction pagefaults should
fail).

Grigori, Tom, can you enlight us about the issue on the URL above. How
can it be triggered?

So after looking at the code in 2.6.14 and current git, I think the
above URL isn't relevant, unless there was a change I missed (which
could totally be possible) that reverted the patch there and fixed that
issue in a different manner.  But since I didn't figure that out until I
had finished researching it again:

Switching hats for a minute, this came from a bug a customer of
MontaVista found, so I can't give out the testcase :(

To repeat what Joakim said back then:
"I think I have figured this out. The first TLB misses that happen at
app startup is Data TLB misses. These will then hit the NULL L1 entry
and end up in do_page_fault() which will populate the L1 entry. But when
you have a very large app that spans more than one L1 entry (16 MB I
think) it may happen that you will have I-TLB Miss first one of the L1
entrys which will make the I-TLB handler bail out to do_page_fault() and
the app craches(SEGV)."

Looking at the patch again, what I don't see is why I talk about fudging
I-TLB Miss at 0x400 when it's I-TLB Error we fudge at being there, but
then get hung up that there can be a slight diff between the two ("This
is because we check bit 4 of SRR1 in both cases, but in the case of an
I-TLB Miss, this bit is always set, and it only indicates a protection
fault on an I-TLB Error.") so instead of 0x1300 jumping to the handler
at 0x400, we treat it like a regular exception so we know where we came
from, and perhaps missed fixing a case somewhere?

-- 
Tom Rini
http://gate.crashing.org/~trini/