Re: [PATCH v5 23/25] arm64/mm: Implement pte_batch_hint()

[Mark Rutland]

On Fri, Feb 02, 2024 at 08:07:54AM +0000, Ryan Roberts wrote:
> When core code iterates over a range of ptes and calls ptep_get() for
> each of them, if the range happens to cover contpte mappings, the number
> of pte reads becomes amplified by a factor of the number of PTEs in a
> contpte block. This is because for each call to ptep_get(), the
> implementation must read all of the ptes in the contpte block to which
> it belongs to gather the access and dirty bits.
> 
> This causes a hotspot for fork(), as well as operations that unmap
> memory such as munmap(), exit and madvise(MADV_DONTNEED). Fortunately we
> can fix this by implementing pte_batch_hint() which allows their
> iterators to skip getting the contpte tail ptes when gathering the batch
> of ptes to operate on. This results in the number of PTE reads returning
> to 1 per pte.
> 
> Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
>  arch/arm64/include/asm/pgtable.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index ad04adb7b87f..353ea67b5d75 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1220,6 +1220,15 @@ static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
>  		__contpte_try_unfold(mm, addr, ptep, pte);
>  }
>  
> +#define pte_batch_hint pte_batch_hint
> +static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
> +{
> +	if (!pte_valid_cont(pte))
> +		return 1;
> +
> +	return CONT_PTES - (((unsigned long)ptep >> 3) & (CONT_PTES - 1));
> +}
> +
>  /*
>   * The below functions constitute the public API that arm64 presents to the
>   * core-mm to manipulate PTE entries within their page tables (or at least this
> -- 
> 2.25.1
>