Re: [PATCH] x86: make text_poke() atomic using fixmap

Mathieu Desnoyers
2009-03-03T17:14:21Z

* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Masami Hiramatsu wrote:
> > Ingo Molnar wrote:
> >> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
> >>
> >>> Ingo Molnar wrote:
> >>>> * Masami Hiramatsu <mhiramat@redhat.com> wrote:
> >>>>
> >>>>> Ingo Molnar wrote:
> >>>>>>>> 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.
> >>>>> Even if the mapping is atomic, text_poke() has to protect pte
> >>>>> from other text_poke()s while changing code.
> >>>>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
> >>>> Well, but text_poke() is not a serializing API to begin with.
> >>>> It's normally used in code patching sequences when we 'know'
> >>>> that there cannot be similar parallel activities. The kprobes
> >>>> usage of text_poke() looks unsafe - and that needs to be fixed.
> >>> Oh, kprobes already prohibited parallel arming/disarming
> >>> by using kprobe_mutex. :-)
> >> yeah, but still the API is somewhat unsafe.
> >
> > Yeah, kprobe_mutex protects text_poke from other kprobes, but
> > not from other text_poke() users...
> >
> >> In any case, you also answered your own question:
> >>
> >>>>> Even if the mapping is atomic, text_poke() has to protect pte
> >>>>> from other text_poke()s while changing code.
> >>>>> AFAIK, atomic-kmap itself doesn't ensure that, does it?
> >> kprobe_mutex does that.
> >
> > Anyway, text_edit_lock ensures that.
> >
> > By the way, I think set_fixmap/clear_fixmap seems simpler than
> > kmap_atomic() variant. Would you think improving kmap_atomic_prot()
> > is better?
>
> Hi Ingo,
>
> Here is the patch which uses fixmaps instead of vmap in text_poke().
> This made the code much simpler than I thought :).
>
> Thanks,
>
> ----
> Use fixmaps instead of vmap/vunmap in text_poke() for avoiding page allocation
> and delayed unmapping.
>
> At the result of above change, text_poke() becomes atomic and can be called
> from stop_machine() etc.
>

It looks great, thanks !

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

> Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> ---
> arch/x86/include/asm/fixmap_32.h | 2 ++
> arch/x86/include/asm/fixmap_64.h | 2 ++
> arch/x86/kernel/alternative.c | 18 ++++++++++++------
> 3 files changed, 16 insertions(+), 6 deletions(-)
>
> Index: linux-2.6/arch/x86/include/asm/fixmap_32.h
> ===================================================================
> --- linux-2.6.orig/arch/x86/include/asm/fixmap_32.h
> +++ linux-2.6/arch/x86/include/asm/fixmap_32.h
> @@ -81,6 +81,8 @@ enum fixed_addresses {
> #ifdef CONFIG_PARAVIRT
> FIX_PARAVIRT_BOOTMAP,
> #endif
> + FIX_TEXT_POKE0, /* reserve 2 pages for text_poke() */
> + FIX_TEXT_POKE1,
> __end_of_permanent_fixed_addresses,
> /*
> * 256 temporary boot-time mappings, used by early_ioremap(),
> Index: linux-2.6/arch/x86/include/asm/fixmap_64.h
> ===================================================================
> --- linux-2.6.orig/arch/x86/include/asm/fixmap_64.h
> +++ linux-2.6/arch/x86/include/asm/fixmap_64.h
> @@ -49,6 +49,8 @@ enum fixed_addresses {
> #ifdef CONFIG_PARAVIRT
> FIX_PARAVIRT_BOOTMAP,
> #endif
> + FIX_TEXT_POKE0, /* reserve 2 pages for text_poke() */
> + FIX_TEXT_POKE1,
> __end_of_permanent_fixed_addresses,
> #ifdef CONFIG_ACPI
> FIX_ACPI_BEGIN,
> Index: linux-2.6/arch/x86/kernel/alternative.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/alternative.c
> +++ linux-2.6/arch/x86/kernel/alternative.c
> @@ -12,7 +12,9 @@
> #include <asm/nmi.h>
> #include <asm/vsyscall.h>
> #include <asm/cacheflush.h>
> +#include <asm/tlbflush.h>
> #include <asm/io.h>
> +#include <asm/fixmap.h>
>
> #define MAX_PATCH_LEN (255-1)
>
> @@ -495,12 +497,13 @@ void *text_poke_early(void *addr, const
> * It means the size must be writable atomically and the address must be aligned
> * in a way that permits an atomic write. It also makes sure we fit on a single
> * page.
> + *
> + * Note: Must be called under text_mutex.
> */
> void *__kprobes text_poke(void *addr, const void *opcode, size_t len)
> {
> unsigned long flags;
> char *vaddr;
> - int nr_pages = 2;
> struct page *pages[2];
> int i;
>
> @@ -513,14 +516,17 @@ void *__kprobes text_poke(void *addr, co
> pages[1] = virt_to_page(addr + PAGE_SIZE);
> }
> BUG_ON(!pages[0]);
> - if (!pages[1])
> - nr_pages = 1;
> - vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
> - BUG_ON(!vaddr);
> + set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
> + if (pages[1])
> + set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
> + vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
> local_irq_save(flags);
> memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
> local_irq_restore(flags);
> - vunmap(vaddr);
> + clear_fixmap(FIX_TEXT_POKE0);
> + if (pages[1])
> + clear_fixmap(FIX_TEXT_POKE1);
> + local_flush_tlb();
> sync_core();
> /* Could also do a CLFLUSH here to speed up CPU recovery; but
> that causes hangs on some VIA CPUs. */
>
> --
> Masami Hiramatsu
>
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
>
> e-mail: mhiramat@redhat.com
>

--
Mathieu Desnoyers
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