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

    * Masami Hiramatsu <mhiramat@redhat.com> wrote:

    >
    >
    > Ingo Molnar wrote:
    > > * Masami Hiramatsu <mhiramat@redhat.com> wrote:
    > >
    > >> Mathieu Desnoyers wrote:
    > >>> * Masami Hiramatsu (mhiramat@redhat.com) 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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