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

[Tom Rini <trini@kernel.crashing.org>]

On Mon, Nov 07, 2005 at 07:14:15PM +0100, Joakim Tjernlund wrote:
> > -----Original Message-----
> > From: Tom Rini [mailto:trini@kernel.crashing.org] 
> > Sent: 07 November 2005 16:52
> > To: Marcelo Tosatti
> > Cc: Joakim Tjernlund; Pantelis Antoniou; Dan Malek; 
> > linuxppc-embedded@ozlabs.org; gtolstolytkin@ru.mvista.com
> > Subject: 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:
> > > > Hi Marcelo
> > > > 
> > > > [SNIP] 
> > > > > 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:
> 
> I wasn't clear enough. What I meant was that the above patch made me
> think and
> the result was that I came up with a simpler fix, the "two exception"
> fix that
> is in current kernels. See
> http://linux.bkbits.net:8080/linux-2.6/diffs/arch/ppc/kernel/head_8xx.S@
> 1.19?nav=index.html|src/.|src/arch|src/arch/ppc|src/arch/ppc/kernel|hist
> /arch/ppc/kernel/head_8xx.S
> It appears this fix has some other issues :(
> 
> How do the other ppc arches do? I am guessing that they don't double
> fault, but bails
> out to do_page_fault from the TLB Miss handler, like 8xx used to do.

Assuming Dan doesn't come up with a more simple & better fix, maybe we
should go back to the original patch I made?

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