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


    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...

    >
    > Ingo
    > --
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    --
    Masami Hiramatsu

    Software Engineer
    Hitachi Computer Products (America) Inc.
    Software Solutions Division

    e-mail: mhiramat@redhat.com



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