Subject: Re: [v2 PATCH 1/2] powerpc32: adds handling of _PAGE_RO

From: Scott Wood <[email-masked]>
Date: Wed, 17 Dec 2014 20:14:28 -0600

On Wed, 2014-12-17 at 10:14 +0100, Christophe Leroy wrote:
> Some powerpc like the 8xx don't have a RW bit in PTE bits but a RO (Read Only) bit.
> This patch implements the handling of a _PAGE_RO flag to be used in place of _PAGE_RW
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> 
> ---
> v2 is a complete rework compared to v1
> 
>  arch/powerpc/include/asm/pgtable-ppc32.h | 11 ++++++-----
>  arch/powerpc/include/asm/pgtable.h       | 10 +++++++---
>  arch/powerpc/include/asm/pte-common.h    | 27 ++++++++++++++++++---------
>  arch/powerpc/mm/gup.c                    |  2 ++
>  arch/powerpc/mm/mem.c                    |  2 +-
>  arch/powerpc/mm/pgtable_32.c             | 24 ++++++++++++++++++++----
>  6 files changed, 54 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/pgtable-ppc32.h b/arch/powerpc/include/asm/pgtable-ppc32.h
> index 543bb8e..64ed9e1 100644
> --- a/arch/powerpc/include/asm/pgtable-ppc32.h
> +++ b/arch/powerpc/include/asm/pgtable-ppc32.h
> @@ -125,7 +125,7 @@ extern int icache_44x_need_flush;
>  #ifndef __ASSEMBLY__
>  
>  #define pte_clear(mm, addr, ptep) \
> -	do { pte_update(ptep, ~_PAGE_HASHPTE, 0); } while (0)
> +	do { pte_update(ptep, ~_PAGE_HASHPTE, _PAGE_RO); } while (0)

Is this really necessary?  It's already clearing the valid bit.

Likewise in several other places that set or check for _PAGE_RO on pages
for which no access is permitted.

> @@ -287,8 +287,9 @@ static inline void huge_ptep_set_wrprotect(struct mm_struct *mm,
>  static inline void __ptep_set_access_flags(pte_t *ptep, pte_t entry)
>  {
>  	unsigned long bits = pte_val(entry) &
> -		(_PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_RW | _PAGE_EXEC);
> -	pte_update(ptep, 0, bits);
> +		(_PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_RW | _PAGE_RO |
> +		 _PAGE_EXEC);
> +	pte_update(ptep, _PAGE_RO, bits);
>  }

You're unconditionally clearing _PAGE_RO, and apparently relying on the
undocumented behavior of pte_update() to clear "clr" before setting
"set".

Instead I'd write this as:

	unsigned long set = pte_val(entry) &
		(_PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_RW | _PAGE_EXEC);
	unsigned long clr = pte_val(entry) & _PAGE_RO;

	pte_update(ptep, clr, set);

-Scott