linux-kernel - Re: [PATCH] powerpc/8xx: use pmd_off() to access a PMD entry in pte

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Date:   Mon, 15 Jun 2020 11:32:42 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH] powerpc/8xx: use pmd_off() to access a PMD entry in
 pte_update()



Le 15/06/2020 à 11:22, Mike Rapoport a écrit :
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> The pte_update() implementation for PPC_8xx unfolds page table from the PGD
> level to access a PMD entry. Since 8xx has only 2-level page table this can
> be simplified with pmd_off() shortcut.
> 
> Replace explicit unfolding with pmd_off() and drop defines of pgd_index()
> and pgd_offset() that are no longer needed.
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>

Nice. When I implemented this function, I was not able to use pmd_ptr()
due to some circular header inclusion (See comment in commit log). Looks
like since the replacement of pmd_ptr() by pmd_off(), it works.

Reviewed-by: Christophe Leroy <christophe.leroy@....fr>

Christophe

> ---
> 
> I think it's powerpc material, but I won't mind if Andrew picks it up :)
> 
> 
>   arch/powerpc/include/asm/nohash/32/pgtable.h | 8 +++-----
>   1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/nohash/32/pgtable.h b/arch/powerpc/include/asm/nohash/32/pgtable.h
> index b56f14160ae5..5a590ceaec14 100644
> --- a/arch/powerpc/include/asm/nohash/32/pgtable.h
> +++ b/arch/powerpc/include/asm/nohash/32/pgtable.h
> @@ -205,10 +205,6 @@ static inline void pmd_clear(pmd_t *pmdp)
>   	*pmdp = __pmd(0);
>   }
>   
> -/* to find an entry in a page-table-directory */
> -#define pgd_index(address)	 ((address) >> PGDIR_SHIFT)
> -#define pgd_offset(mm, address)	 ((mm)->pgd + pgd_index(address))
> -
>   /*
>    * PTE updates. This function is called whenever an existing
>    * valid PTE is updated. This does -not- include set_pte_at()
> @@ -230,6 +226,8 @@ static inline void pmd_clear(pmd_t *pmdp)
>    * For other page sizes, we have a single entry in the table.
>    */
>   #ifdef CONFIG_PPC_8xx
> +static pmd_t *pmd_off(struct mm_struct *mm, unsigned long addr);
> +
>   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)
>   {
> @@ -237,7 +235,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
>   	pte_basic_t old = pte_val(*p);
>   	pte_basic_t new = (old & ~(pte_basic_t)clr) | set;
>   	int num, i;
> -	pmd_t *pmd = pmd_offset(pud_offset(p4d_offset(pgd_offset(mm, addr), addr), addr), addr);
> +	pmd_t *pmd = pmd_off(mm, addr);
>   
>   	if (!huge)
>   		num = PAGE_SIZE / SZ_4K;
>