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

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Date:	Fri, 20 Feb 2009 13:00:31 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
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


On Fri, 20 Feb 2009, Ingo Molnar wrote:

> 
> * 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.

I'm not saying that was the reason kprobes did it that way. I was just 
answering your question about how kprobes does it today ;-)

Perhaps it may be more efficient to convert kprobes to use the 
set_memory_rw/ro API now.

-- Steve

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