linux-kernel - Re: [PATCH 1/3] powerpc/nohash: Fix __ptep_set_access_flags() and ptep_set

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Message-Id: <1635308538.6vye6lbbh8.astroid@bobo.none>
Date:   Wed, 27 Oct 2021 14:23:42 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/3] powerpc/nohash: Fix __ptep_set_access_flags() and
 ptep_set_wrprotect()

Excerpts from Christophe Leroy's message of October 26, 2021 3:39 pm:
> Commit 26973fa5ac0e ("powerpc/mm: use pte helpers in generic code")
> changed those two functions to use pte helpers to determine which
> bits to clear and which bits to set.
> 
> This change was based on the assumption that bits to be set/cleared
> are always the same and can be determined by applying the pte
> manipulation helpers on __pte(0).
> 
> But on platforms like book3e, the bits depend on whether the page
> is a user page or not.
> 
> For the time being it more or less works because of _PAGE_EXEC being
> used for user pages only and exec right being set at all time on
> kernel page. But following patch will clean that and output of
> pte_mkexec() will depend on the page being a user or kernel page.
> 
> Instead of trying to make an even more complicated helper where bits
> would become dependent on the final pte value, come back to a more
> static situation like before commit 26973fa5ac0e ("powerpc/mm: use
> pte helpers in generic code"), by introducing an 8xx specific
> version of __ptep_set_access_flags() and ptep_set_wrprotect().

What is this actually fixing? Does it change anything itself, or
just a preparation patch?

Thanks,
Nick

> 
> Fixes: 26973fa5ac0e ("powerpc/mm: use pte helpers in generic code")
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> v3: No change
> v2: New
> ---
>  arch/powerpc/include/asm/nohash/32/pgtable.h | 17 +++++++--------
>  arch/powerpc/include/asm/nohash/32/pte-8xx.h | 22 ++++++++++++++++++++
>  2 files changed, 30 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/nohash/32/pgtable.h b/arch/powerpc/include/asm/nohash/32/pgtable.h
> index 34ce50da1850..11c6849f7864 100644
> --- a/arch/powerpc/include/asm/nohash/32/pgtable.h
> +++ b/arch/powerpc/include/asm/nohash/32/pgtable.h
> @@ -306,30 +306,29 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm, unsigned long addr,
>  }
>  
>  #define __HAVE_ARCH_PTEP_SET_WRPROTECT
> +#ifndef ptep_set_wrprotect
>  static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr,
>  				      pte_t *ptep)
>  {
> -	unsigned long clr = ~pte_val(pte_wrprotect(__pte(~0)));
> -	unsigned long set = pte_val(pte_wrprotect(__pte(0)));
> -
> -	pte_update(mm, addr, ptep, clr, set, 0);
> +	pte_update(mm, addr, ptep, _PAGE_RW, 0, 0);
>  }
> +#endif
>  
> +#ifndef __ptep_set_access_flags
>  static inline void __ptep_set_access_flags(struct vm_area_struct *vma,
>  					   pte_t *ptep, pte_t entry,
>  					   unsigned long address,
>  					   int psize)
>  {
> -	pte_t pte_set = pte_mkyoung(pte_mkdirty(pte_mkwrite(pte_mkexec(__pte(0)))));
> -	pte_t pte_clr = pte_mkyoung(pte_mkdirty(pte_mkwrite(pte_mkexec(__pte(~0)))));
> -	unsigned long set = pte_val(entry) & pte_val(pte_set);
> -	unsigned long clr = ~pte_val(entry) & ~pte_val(pte_clr);
> +	unsigned long set = pte_val(entry) &
> +			    (_PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_RW | _PAGE_EXEC);
>  	int huge = psize > mmu_virtual_psize ? 1 : 0;
>  
> -	pte_update(vma->vm_mm, address, ptep, clr, set, huge);
> +	pte_update(vma->vm_mm, address, ptep, 0, set, huge);
>  
>  	flush_tlb_page(vma, address);
>  }
> +#endif
>  
>  static inline int pte_young(pte_t pte)
>  {
> diff --git a/arch/powerpc/include/asm/nohash/32/pte-8xx.h b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> index fcc48d590d88..1a89ebdc3acc 100644
> --- a/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> +++ b/arch/powerpc/include/asm/nohash/32/pte-8xx.h
> @@ -136,6 +136,28 @@ static inline pte_t pte_mkhuge(pte_t pte)
>  
>  #define pte_mkhuge pte_mkhuge
>  
> +static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, pte_t *p,
> +				     unsigned long clr, unsigned long set, int huge);
> +
> +static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
> +{
> +	pte_update(mm, addr, ptep, 0, _PAGE_RO, 0);
> +}
> +#define ptep_set_wrprotect ptep_set_wrprotect
> +
> +static inline void __ptep_set_access_flags(struct vm_area_struct *vma, pte_t *ptep,
> +					   pte_t entry, unsigned long address, int psize)
> +{
> +	unsigned long set = pte_val(entry) & (_PAGE_DIRTY | _PAGE_ACCESSED | _PAGE_EXEC);
> +	unsigned long clr = ~pte_val(entry) & _PAGE_RO;
> +	int huge = psize > mmu_virtual_psize ? 1 : 0;
> +
> +	pte_update(vma->vm_mm, address, ptep, clr, set, huge);
> +
> +	flush_tlb_page(vma, address);
> +}
> +#define __ptep_set_access_flags __ptep_set_access_flags
> +
>  static inline unsigned long pgd_leaf_size(pgd_t pgd)
>  {
>  	if (pgd_val(pgd) & _PMD_PAGE_8M)
> -- 
> 2.31.1
> 
>