Re: [RFC][PATCH] x86: make text_poke() atomic

[Ingo Molnar]

* Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> 
> 
> Ingo Molnar wrote:
> > * Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > 
> >> Mathieu Desnoyers wrote:
> >>> * Masami Hiramatsu (mhiramat@xxxxxxxxxx) wrote:
> >>>> Index: linux-2.6/init/main.c
> >>>> ===================================================================
> >>>> --- linux-2.6.orig/init/main.c
> >>>> +++ linux-2.6/init/main.c
> >>>> @@ -676,6 +676,9 @@ asmlinkage void __init start_kernel(void
> >>>>  	taskstats_init_early();
> >>>>  	delayacct_init();
> >>>>
> >>>> +#ifdef CONFIG_X86
> >>>> +	text_poke_init();
> >>>> +#endif
> >>> All good, except this above. There should be an empty text_poke_init()
> >>> in some header file, and an implementation for the X86 arch rather than
> >>> a ifdef in init/main.c.
> >> Hmm, I'd rather use __weak function instead of defining it in some header
> >> files, because text_poke() and alternatives exist only on x86.
> >>
> >> I know that we need to discuss cross modifying code on x86 with
> >> Arjan or other Intel engineers. This patch may still be useful
> >> for removing unnecessary vm_area allocation in text_poke().
> >>
> >> Thank you,
> >>
> >> ---
> >>
> >> Use map_vm_area() instead of vmap() in text_poke() for 
> >> avoiding page allocation and delayed unmapping, and call 
> >> vunmap_page_range() and local_flush_tlb() directly because 
> >> this mapping is temporary and local.
> >>
> >> At the result of above change, text_poke() becomes atomic and 
> >> can be called from stop_machine() etc.
> > 
> > That looks like a good fix in itself - see a few minor details 
> > below.
> 
> Thank you for review,
> 
> > 
> > (Note, i could not try your patch because it has widespread 
> > whitespace damage - please watch out for this for future 
> > patches.)
> 
> Oops, it was my mis-setting...
> 
> > 
> >> +static struct vm_struct *text_poke_area[2];
> >> +static DEFINE_SPINLOCK(text_poke_lock);
> >> +
> >> +void __init text_poke_init(void)
> >> +{
> >> +	text_poke_area[0] = get_vm_area(PAGE_SIZE, VM_ALLOC);
> >> +	text_poke_area[1] = get_vm_area(2 * PAGE_SIZE, VM_ALLOC);
> >> +	BUG_ON(!text_poke_area[0] || !text_poke_area[1]);
> > 
> > BUG_ON() for non-100%-essential init code is a no-no. Please 
> > change it to WARN_ON() so that people have a chance to report i.
> 
> Sure.
> 
> > 
> > Also, i think all these vma complications came from the decision 
> > to use vmap - and vmap enhancements in .29 complicated this 
> > supposedly-simple interface.
> > 
> > So perhaps another approach to (re-)consider would be to go back 
> > to atomic fixmaps here. It spends 3 slots but that's no big 
> > deal.
> 
> Oh, it's a good idea! fixmaps must make it simpler.
> 
> > 
> > In exchange it will be conceptually simpler, and will also scale 
> > much better than a global spinlock. What do you think?
> 
> I think even if I use fixmaps, we have to use a spinlock to protect
> the fixmap area from other threads...

that's why i suggested to use an atomic-kmap, not a fixmap.

	Ingo
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