Re: [PATCH -tip 4/4] Atomic text_poke() with fixmap

[Ingo Molnar <mingo@elte.hu>]

* Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> * Masami Hiramatsu (mhiramat@redhat.com) wrote:
> > 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.
> > 
> > Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
> > Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
> > Cc: Ingo Molnar <mingo@elte.hu>
> > ---
> >  arch/x86/include/asm/fixmap.h |    2 ++
> >  arch/x86/kernel/alternative.c |   24 +++++++++++++++---------
> >  2 files changed, 17 insertions(+), 9 deletions(-)
> > 
> > Index: linux-2.6-tip/arch/x86/kernel/alternative.c
> > ===================================================================
> > --- linux-2.6-tip.orig/arch/x86/kernel/alternative.c
> > +++ linux-2.6-tip/arch/x86/kernel/alternative.c
> > @@ -13,7 +13,9 @@
> >  #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)
> > 
> > @@ -505,15 +507,16 @@ 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;
> > 
> > -	might_sleep();
> >  	if (!core_kernel_text((unsigned long)addr)) {
> >  		pages[0] = vmalloc_to_page(addr);
> >  		pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
> > @@ -523,14 +526,17 @@ 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);
> > -	local_irq_disable();
> > +	set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
> 
> Can the set_fixmap/clear_fixmap/local_flush_tlb be called 
> within local_irq_save ? If yes, that would be better, 
> especially for the SMP alternatives code, which would rely on 
> interrupt disabling in text_poke for consistency (the mutex is 
> not needed there).

yes, it is atomic.

	Ingo