linux-kernel - Re: [PATCH 2/5] ftrace, x86: make kernel text writable only for conversions

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Date:	Fri, 20 Feb 2009 18:55:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/5] ftrace, x86: make kernel text writable only for
	conversions


* Steven Rostedt <rostedt@...dmis.org> wrote:

> > > +/* used by ftrace */
> > > +void set_kernel_text_rw(void)
> > 
> > i'd leave out the 'used by ftrace' bit - more uses might 
> > arise. How does kprobes get around readonly pages, it uses 
> > these APIs too, right?
> 
> kprobes uses text_poke. text_poke uses vmap to create its own 
> page table pointers to the text memory, does the modification 
> and then removes the pointers. This is quite heavy weight and 
> since ftrace needs to modify 10s of thousands of areas, 
> converting all of kernel text page tables is much more 
> efficient.
> 
> Note, if kprobes did change the original page tables, then it 
> too would have hit the split_large_page bug too.

i think the real reason is that kprobes was written before we 
had this more flexible and more usable CPA code.

	Ingo
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