Subject: Re: [PATCH v6 6/8] x86/module: prepare module loading for ROX allocations of text

From: Peter Zijlstra <[email-masked]>
Date: Thu, 17 Oct 2024 11:35:15 +0200

On Wed, Oct 16, 2024 at 05:01:28PM -0400, Steven Rostedt wrote:
> On Wed, 16 Oct 2024 15:24:22 +0300
> Mike Rapoport <[email-masked]> wrote:
> 
> > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > index 8da0e66ca22d..b498897b213c 100644
> > --- a/arch/x86/kernel/ftrace.c
> > +++ b/arch/x86/kernel/ftrace.c
> > @@ -118,10 +118,13 @@ ftrace_modify_code_direct(unsigned long ip, const char *old_code,
> >  		return ret;
> >  
> >  	/* replace the text with the new text */
> > -	if (ftrace_poke_late)
> > +	if (ftrace_poke_late) {
> >  		text_poke_queue((void *)ip, new_code, MCOUNT_INSN_SIZE, NULL);
> > -	else
> > -		text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
> > +	} else {
> > +		mutex_lock(&text_mutex);
> > +		text_poke((void *)ip, new_code, MCOUNT_INSN_SIZE);
> > +		mutex_unlock(&text_mutex);
> > +	}
> >  	return 0;
> >  }
> 
> So this slows down the boot by over 30ms. That may not sound like much, but
> we care very much about boot times. This code is serialized with boot and
> runs whenever ftrace is configured in the kernel. The way I measured this,
> was that I added:
> 

> If this is only needed for module load, can we at least still use the
> text_poke_early() at boot up?

Right, so I don't understand why this is needed at all.
ftrace_module_init() runs before complete_formation() which normally
switches to ROX, as such ftrace should be able to continue to do direct
modifications here.

Which reminds me, at some point I did patches adding a
MODULE_STATE_UNFORMED callback in order for static_call / jump_label to
be able to avoid the expensive patching on module load as well (arguably
ftrace should be using that too, instead of a custom callback).