Re: [PATCH] slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN

[Catalin Marinas <catalin.marinas@arm.com>]

On Sat, Aug 08, 2026 at 12:56:22PM +0200, Karl Mehltretter wrote:
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 32c9f8ed7ae20..0798a714da87f 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -259,13 +259,16 @@ enum _slab_flag_bits {
>  
>  /*
>   * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
> + * It satisfies the alignment promised by __assume_kmalloc_alignment
> + * and keeps the historic value 16 where that is already aligned.
>   *
>   * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault.
>   *
>   * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can.
>   * Both make kfree a no-op.
>   */
> -#define ZERO_SIZE_PTR ((void *)16)
> +#define ZERO_SIZE_PTR ((void *)(ARCH_KMALLOC_MINALIGN > 16 ? \
> +				ARCH_KMALLOC_MINALIGN : 16))
>  
>  #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
>  				(unsigned long)ZERO_SIZE_PTR)
> @@ -622,6 +625,12 @@ static inline bool kmem_dump_obj(void *object) { return false; }
>  #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE)
>  #endif
>  
> +/*
> + * Keep ZERO_SIZE_PTR below PAGE_SIZE and the low pointer poison values.
> + * 128 is the largest in-tree ARCH_KMALLOC_MINALIGN.
> + */
> +static_assert(ARCH_KMALLOC_MINALIGN <= 128);

I'm not sure we should bother with this, or at least make it strictly
less than PAGE_SIZE since 128 doesn't have any meaning for the slab
allocator.

Otherwise,

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>