linux-kernel - Re: [PATCH 1/5] ftrace: use module notifier for function tracer

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Message-ID: <49F0CA5C.8090409@redhat.com>
Date:	Thu, 23 Apr 2009 16:06:52 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Anders Kaseorg <andersk@....EDU>
CC:	Tim Abbott <tabbott@....EDU>, Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jeffrey B Arnold <jbarnold@....EDU>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH 1/5] ftrace: use module notifier for function tracer

Anders Kaseorg wrote:
> On Wed, 22 Apr 2009, Masami Hiramatsu wrote:
>> Could you also add "use text_poke() on x86" to your plan? :-)
> 
> That should be possible now that it is usable inside stop_machine().  It 
> would be nicer to have an API that isn’t x86-specific, though.

Now, text_poke() become atomic and can be called inside stop_machine().
I agree with that text_poke() is currently implemented only on x86,
but adding a generic text_poke() which just do memcpy() is easy.

> Another issue is that Ksplice supports patching rodata as well as text, 
> and text_poke() does not support changes bigger than PAGE_SIZE.  Though 
> perhaps text_poke() is not the right function for rodata patches anyway.

PAGE_SIZE limitation is not a problem, because you can call text_poke()
repeatedly.
But indeed, handling rodata in text_poke() is a bit odd...

I assume that you are using alias pages (mapping ro-pages to rw-area) in
Ksplice, and if so, I think we can make it as a generic function and
share it with text_poke().

>> Hmm, I can't agree that we allow module to modify kernel text...
>> Is that possible to separate kernel-text swapping routine from Ksplice
>> module? In that case, we don't need to export text_mutex.
> 
> No, it’s not enough for Ksplice to lock the kernel text only while 
> actively swapping it.  We also need to prevent changes to the kernel text 
> while Ksplice is doing run-pre matching to ensure safety.  This means that 
> Ksplice wants to hold text_mutex for essentially the entire time it’s 
> running.

Then, why can't you move that "matching" routine into the kernel too? :-)

Thank you,


-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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