Re: [RFC PATCH v2 08/11] ARM64: mm: Swap PTE_FILE and PTE_PROT_NONE bits.

[Will Deacon <will.deacon@arm.com>]

On Wed, May 08, 2013 at 10:52:40AM +0100, Steve Capper wrote:
> Under ARM64, PTEs can be broadly categorised as follows:
>    - Present and valid: Bit #0 is set. The PTE is valid and memory
>      access to the region may fault.
> 
>    - Present and invalid: Bit #0 is clear and bit #1 is set.
>      Represents present memory with PROT_NONE protection. The PTE
>      is an invalid entry, and the user fault handler will raise a
>      SIGSEGV.
> 
>    - Not present (file): Bits #0 and #1 are clear, bit #2 is set.
>      Memory represented has been paged out. The PTE is an invalid
>      entry, and the fault handler will try and re-populate the
>      memory where necessary.
> 
> Huge PTEs are block descriptors that have bit #1 clear. If we wish
> to represent PROT_NONE huge PTEs we then run into a problem as
> there is no way to distinguish between regular and huge PTEs if we
> set bit #1.
> 
> As huge PTEs are always present, the meaning of bits #1 and #2 can
> be swapped for invalid PTEs. This patch swaps the PTE_FILE and
> PTE_PROT_NONE constants, allowing us to represent PROT_NONE huge
> PTEs.
> 
> Signed-off-by: Steve Capper <steve.capper@linaro.org>
> ---
>  arch/arm64/include/asm/pgtable.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index b1a1b59..e245260 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -25,8 +25,8 @@
>   * Software defined PTE bits definition.
>   */
>  #define PTE_VALID		(_AT(pteval_t, 1) << 0)
> -#define PTE_PROT_NONE		(_AT(pteval_t, 1) << 1)	/* only when !PTE_VALID */
> -#define PTE_FILE		(_AT(pteval_t, 1) << 2)	/* only when !pte_present() */
> +#define PTE_FILE		(_AT(pteval_t, 1) << 1)	/* only when !pte_present() */
> +#define PTE_PROT_NONE		(_AT(pteval_t, 1) << 2)	/* only when !PTE_VALID */
>  #define PTE_DIRTY		(_AT(pteval_t, 1) << 55)
>  #define PTE_SPECIAL		(_AT(pteval_t, 1) << 56)
>  
> @@ -306,8 +306,8 @@ extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
>  
>  /*
>   * Encode and decode a file entry:
> - *	bits 0-1:	present (must be zero)
> - *	bit  2:		PTE_FILE
> + *	bits 0 & 2:	present (must be zero)
> + *	bit  1:		PTE_FILE
>   *	bits 3-63:	file offset / PAGE_SIZE
>   */
>  #define pte_file(pte)		(pte_val(pte) & PTE_FILE)

Can you update the comment describing swp entries too please? I *think* the
__SWP_* defines can remain untouched, but the comment is now wrong.

Will

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