linux-kernel - Re: [v2 PATCH 1/2] powerpc32: adds handling of _PAGE

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Message-ID: <1418901787.5581.131.camel@freescale.com>
Date:	Thu, 18 Dec 2014 05:23:07 -0600
From:	Scott Wood <scottwood@...escale.com>
To:	leroy christophe <christophe.leroy@....fr>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	"Joakim Tjernlund" <joakim.tjernlund@...nsmode.se>
Subject: Re: [v2 PATCH 1/2] powerpc32: adds handling of _PAGE_RO

On Thu, 2014-12-18 at 08:11 +0100, leroy christophe wrote:
> Le 18/12/2014 03:14, Scott Wood a écrit :
> > 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@....fr>
> >>
> >> ---
> >> 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.

Yes, sorry.

-Scott


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