Re: [RFC PATCH 02/34] ARM: mm: make 2-level pgd_t a scalar

[Pedro Falcato <pfalcato@suse.de>]

On Mon, Jul 13, 2026 at 02:55:41PM +0100, Yeoreum Yun wrote:
> From: "David Hildenbrand (Arm)" <david@kernel.org>
> 
> We don't want pgd_t to be an array, as it prohibits returning it from a
> function, like pgdp_get().
> 
> So let's just use an u64, and extract the right 32bit value in
> pgd_val().
> 
> Leave the STRICT_MM_TYPECHECKS case alone for now.

I have to ask: is there a good reason for the STRICT_MM_TYPECHECKS ifdef?

I see the compiler has an awkward time returning a u64 struct (see
https://godbolt.org/z/qejbv6j9a), but if this doesn't work maybe we should
get rid of the STRICT_MM_TYPECHECKS stuff? I seriously doubt anyone is
purposefully toggling it on for testing from time to time.

If STRICT_MM_TYPEDEFS's worse codegen doesn't matter then maybe we should
permanently toggle it on.
> 
> As an alternative, we could use the STRICT_MM_TYPECHECKS approach here
> as well, but using an u64 looks conceptually cleaner, even though
> pgd_val() gets a bit more involved.
> 
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
>  arch/arm/include/asm/pgtable-2level-types.h | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/include/asm/pgtable-2level-types.h b/arch/arm/include/asm/pgtable-2level-types.h
> index 650e793f41429..02052cef9437a 100644
> --- a/arch/arm/include/asm/pgtable-2level-types.h
> +++ b/arch/arm/include/asm/pgtable-2level-types.h
> @@ -25,7 +25,7 @@ typedef struct { pteval_t pgprot; } pgprot_t;
>  
>  #define pte_val(x)      ((x).pte)
>  #define pmd_val(x)      ((x).pmd)
> -#define pgd_val(x)	((x).pgd[0])
> +#define pgd_val(x)      ((x).pgd[0])
>  #define pgprot_val(x)   ((x).pgprot)
>  
>  #define __pte(x)        ((pte_t) { (x) } )
> @@ -36,14 +36,21 @@ typedef struct { pteval_t pgprot; } pgprot_t;
>  /*
>   * .. while these make it easier on the compiler
>   */
> +typedef u64 pgdval_t;
> +
>  typedef pteval_t pte_t;
>  typedef pmdval_t pmd_t;
> -typedef pmdval_t pgd_t[2];
> +typedef pgdval_t pgd_t;
>  typedef pteval_t pgprot_t;
>  
>  #define pte_val(x)      (x)
>  #define pmd_val(x)      (x)
> -#define pgd_val(x)	((x)[0])
> +
> +static inline pmdval_t pgd_val(pgd_t pgd)
> +{
> +	return (*(pmdval_t (*)[2])&pgd)[0];

Ugh. This isn't correct C code. It only works because the kernel passes
-fno-strict-aliasing. I would recommend either forcing a struct here, or
using a u64 with bitmasks/shifts.

-- 
Pedro