Date:	Wed, 9 Jan 2008 03:17:37 +0000 (GMT)
From:	Dave Airlie <airlied@...ux.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	torvalds@...ux-foundation.org, hch@....de, ak@...e.de, pq@....fi,
	jbeulich@...ell.com, mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "x86: optimize page faults like all other
 achitectures and kill notifier cruft"


> On Wed, 9 Jan 2008 02:34:46 +0000 (GMT) Dave Airlie <airlied@...ux.ie> wrote:
> 
> > 
> > [This an initial RFC but I'd like to have this patch in before 2.6.24 goes 
> > final as it really breaks this useful feature]
> > 
> > mmiotrace the MMIO access tracer used to reverse engineer binary blobs
> > used this notifier interface and is planned on being pushed upstream.
> > 
> > Having users able to just use the tracer module without having to rebuild 
> > their kernel to add in a page fault handler hack means we get a lot 
> > greater coverage for reverse engineering efforts.
> 
> Sorry, but that's a really really small benefit.  This very small number of
> fairly (or very) technical users will be able to work out a way of getting
> this to work in 2.6.24.  And in 2.6.25 with a merged mmiotrace we can do
> something different.

mmiotrace isn't targetted at fairly or technical users, its whole 
usefulness is that you don't need a kernel re-build, the distro kernels 
all contain enough support for us to just get a user to grab mmiotrace, 
run make and get a trace.... so in my eyes this a major feature regression 
to have to go back to custom kernel builds...

> It's a modest convenience to a very small number of people.  And the cost? 
> Multiple functions calls and multiple cachelines hit for every pagefault
> on, what?  Tens of millions of machines?

Which has been happening for how many months? perhaps if we merge 
mmiotrace in 2.6.25 we can clean up this function, otherwise I just count 
it as a feature regression...

> pagefault it populates a struct on the stack, passes that around for a
> while, does a bit of RCU stuff only to find that there was nothing to do. 
> Surely we should at least be doing something along the lines of
> 
> 	if (unlikely(notify_page_fault_chain.notifier_call != NULL)) {
> 		all that crap
> 	}
> 
> 
> But that's all speculation.  Has anyone actually measured the pagefault
> latency impact of this change?
> 
> > +/*
> > + * These are only here because kprobes.c wants them to implement a
> > + * blatant layering violation.  Will hopefully go away soon once all
> > + * architectures are updated.
> > + */
> > +static inline int register_page_fault_notifier(struct notifier_block *nb)
> > +{
> > +	return 0;
> > +}
> > +static inline int unregister_page_fault_notifier(struct notifier_block *nb)
> > +{
> > +	return 0;
> > +}
> > +
> 
> And this doesn't look very good either.  For how long did this fixme remain
> unfixed?
> 
> 
> So I'd suggest that we leave things as they are for 2.6.24 - mmiotrace
> people will work something out, I'm sure.  For 2.6.25 if we merge mmiotrace
> we can look at doing something which is vaguely efficient and tasteful.
> 

I just reverted Christophs patch I didn't try and work out if the old code 
had problems no one has fixed...

So all distros with 2.6.24 kernels are useless to mmiotrace I don't see 
why leaving things as is until a suitable replacement mechanism can be 
used.. I've heard others give out about this also madwifi and SuSE kernel 
folks...

Dave.
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