[PATCH v7 11/15] powerpc/code-patching: Avoid r/w mapping of the zero page

[Mukesh Kumar Chaurasiya]

On Fri, May 29, 2026 at 05:02:02PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The only remaining use of map_patch_area() is mapping the zero page, and
> immediately unmapping it again so that the intermediate page table
> levels are all guaranteed to be populated.
> 
> The use of the zero page here is completely arbitrary, and not harmful
> per se, but currently, it creates a writable mapping, and does so in a
> manner that requires that the empty_zero_page[] symbol is not
> const-qualified.
> 
> Given that this is about to change, and that map_patch_area() now never
> maps anything other than the zero page, let's simplify the code and
> - remove the helpers and call [un]map_kernel_page() directly
> - take the PA of empty_zero_page directly
> - create a read-only temporary mapping.
> 
> This allows empty_zero_page[] to be repainted as const u8[] in a
> subsequent patch, without making substantial changes to this code
> patching logic.
> 
> Cc: Madhavan Srinivasan <maddy at linux.ibm.com>
> Cc: Michael Ellerman <mpe at ellerman.id.au>
> Cc: Nicholas Piggin <npiggin at gmail.com>
> Cc: "Christophe Leroy (CS GROUP)" <chleroy at kernel.org>
> Link: https://lore.kernel.org/all/20260520085423.485402-1-ardb@kernel.org/
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/powerpc/lib/code-patching.c | 52 +-------------------
>  1 file changed, 2 insertions(+), 50 deletions(-)
> 
> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> index f84e0337cc02..44ff9f684bef 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -60,9 +60,6 @@ struct patch_context {
>  
>  static DEFINE_PER_CPU(struct patch_context, cpu_patching_context);
>  
> -static int map_patch_area(void *addr, unsigned long text_poke_addr);
> -static void unmap_patch_area(unsigned long addr);
> -
>  static bool mm_patch_enabled(void)
>  {
>  	return IS_ENABLED(CONFIG_SMP) && radix_enabled();
> @@ -117,11 +114,11 @@ static int text_area_cpu_up(unsigned int cpu)
>  
>  	// Map/unmap the area to ensure all page tables are pre-allocated
>  	addr = (unsigned long)area->addr;
> -	err = map_patch_area(empty_zero_page, addr);
> +	err = map_kernel_page(addr, __pa_symbol(empty_zero_page), PAGE_KERNEL_RO);
>  	if (err)
>  		return err;
>  
> -	unmap_patch_area(addr);
> +	unmap_kernel_page(addr);
>  
>  	this_cpu_write(cpu_patching_context.area, area);
>  	this_cpu_write(cpu_patching_context.addr, addr);
> @@ -233,51 +230,6 @@ static unsigned long get_patch_pfn(void *addr)
>  		return __pa_symbol(addr) >> PAGE_SHIFT;
>  }
>  
> -/*
> - * This can be called for kernel text or a module.
> - */
> -static int map_patch_area(void *addr, unsigned long text_poke_addr)
> -{
> -	unsigned long pfn = get_patch_pfn(addr);
> -
> -	return map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL);
> -}
> -
> -static void unmap_patch_area(unsigned long addr)
> -{
> -	pte_t *ptep;
> -	pmd_t *pmdp;
> -	pud_t *pudp;
> -	p4d_t *p4dp;
> -	pgd_t *pgdp;
> -
> -	pgdp = pgd_offset_k(addr);
> -	if (WARN_ON(pgd_none(*pgdp)))
> -		return;
> -
> -	p4dp = p4d_offset(pgdp, addr);
> -	if (WARN_ON(p4d_none(*p4dp)))
> -		return;
> -
> -	pudp = pud_offset(p4dp, addr);
> -	if (WARN_ON(pud_none(*pudp)))
> -		return;
> -
> -	pmdp = pmd_offset(pudp, addr);
> -	if (WARN_ON(pmd_none(*pmdp)))
> -		return;
> -
> -	ptep = pte_offset_kernel(pmdp, addr);
> -	if (WARN_ON(pte_none(*ptep)))
> -		return;
> -
> -	/*
> -	 * In hash, pte_clear flushes the tlb, in radix, we have to
> -	 */
> -	pte_clear(&init_mm, addr, ptep);
> -	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -}
> -
>  static int __do_patch_mem_mm(void *addr, unsigned long val, bool is_dword)
>  {
>  	int err;
> -- 
> 2.54.0.823.g6e5bcc1fc9-goog
> 
I don't see any functional change.

Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras at gmail.com>