linux-kernel - Re: [PATCH] Revert "x86: optimize page faults like all other achitectures and kill notifier cruft"

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Date:	Wed, 9 Jan 2008 11:41:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	hch@....de, airlied@...ux.ie, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, ak@...e.de, pq@....fi,
	jbeulich@...ell.com, linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [PATCH] Revert "x86: optimize page faults like all other
	achitectures and kill notifier cruft"

(kprobes folks Cc:-ed)

* David Miller <davem@...emloft.net> wrote:

> From: Christoph Hellwig <hch@....de>
> Date: Wed, 9 Jan 2008 08:19:45 +0100
> 
> > On Wed, Jan 09, 2008 at 03:55:20AM +0000, Dave Airlie wrote:
> > > now because Linus said send him a patch to revert regressions rather than 
> > > just complain,
> > 
> > this is not a regression by any definition.  You were abusing 
> > exported symbols for out of tree junk, so you'll lose.
> 
> And furthermore, they don't even need it, use a kprobe.

i agree. There a few practical complication on x86: the do_page_fault() 
function is currently excluded from kprobe probing, for recursion 
reasons. handle_mm_fault() can be probed OTOH - but that does not catch 
vmalloc()-ed faults. The middle of do_page_fault() [line 348] should 
work better [the point after notify_page_fault()] - but it's usually 
more fragile to insert probes to such middle-of-the-function places.

So probing pagefaults is not as easy as it should/could be. We should 
put a practical NOP marker to around line 348, to make it easier (and 
faster) for systemtap to probe there.

(__kprobes is a highly confusing newspeak name btw - it should be 
__noprobe instead.)

	Ingo
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