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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* 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).

Mathieu

> +	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_enable();
> -	vunmap(vaddr);
> +	local_irq_restore(flags);
> +	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. */
> Index: linux-2.6-tip/arch/x86/include/asm/fixmap.h
> ===================================================================
> --- linux-2.6-tip.orig/arch/x86/include/asm/fixmap.h
> +++ linux-2.6-tip/arch/x86/include/asm/fixmap.h
> @@ -111,6 +111,8 @@ 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_PROVIDE_OHCI1394_DMA_INIT
>  	FIX_OHCI1394_BASE,
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@redhat.com
> 

-- 
Mathieu Desnoyers
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