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

[leroy christophe]

Le 18/12/2014 03:14, Scott Wood a Ãcrit :
> 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 <christophe.leroy@xxxxxx>
> >
> > ---
> > 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;
Don't you mean ?

	unsigned long clr = ~pte_val(entry) & _PAGE_RO;

Because, we want to clear _PAGE_RO when _PAGE_RO is not set in entry.

Christophe

> 	pte_update(ptep, clr, set);
>
> -Scott

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