linux-kernel - Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)

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Date:	Fri, 22 Sep 2006 18:53:52 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	Martin Bligh <mbligh@...gle.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	prasanna@...ibm.com, Andrew Morton <akpm@...l.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers 0.5 for Linux 2.6.17 (with probe management)


* Mathieu Desnoyers <compudj@...stal.dyndns.org> wrote:

> > > Then you lose the ability to trace in-kernel minor page faults.
> > 
> > that's wrong, minor pagefaults go through __handle_mm_fault() just as 
> > much.
> > 
> 
> Hi Ingo,
> 
> On a 2.6.17 kernel tree :

> It seems like a shortcut path that will never call __handle_mm_fault. 
> This path is precisely used to handle vmalloc faults.

yes, but you said "minor fault", not "vmalloc fault".

minor faults are the things that happen when a task does read-after-COW 
or read-mmap-ed-pagecache-page, and they very much go through 
__handle_mm_fault().

vmalloc faults are extremely rare, x86-specific and they are a pure 
kernel-internal matter. (I'd never want to trace them, especially if it 
pushes tracepoints into every architecture's page fault handler. I 
implemented the initial version of them IIRC, but my memory fails 
precisely why. I think it was 4:4 related, but i'm unsure.)

(i now realize that above you said "in-kernel minor faults" - under that 
you meant vmalloc faults?)

	Ingo
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