Re: [PATCH v2 3/5] mm: Default implementation of arch_wants_pte_order()

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 7/3/2023 9:53 PM, Ryan Roberts wrote:
> arch_wants_pte_order() can be overridden by the arch to return the
> preferred folio order for pte-mapped memory. This is useful as some
> architectures (e.g. arm64) can coalesce TLB entries when the physical
> memory is suitably contiguous.
> 
> The first user for this hint will be FLEXIBLE_THP, which aims to
> allocate large folios for anonymous memory to reduce page faults and
> other per-page operation costs.
> 
> Here we add the default implementation of the function, used when the
> architecture does not define it, which returns the order corresponding
> to 64K.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  include/linux/pgtable.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index a661a17173fa..f7e38598f20b 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -13,6 +13,7 @@
>  #include <linux/errno.h>
>  #include <asm-generic/pgtable_uffd.h>
>  #include <linux/page_table_check.h>
> +#include <linux/sizes.h>
>  
>  #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
>  	defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
> @@ -336,6 +337,18 @@ static inline bool arch_has_hw_pte_young(void)
>  }
>  #endif
>  
> +#ifndef arch_wants_pte_order
> +/*
> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
> + * to be at least order-2.
> + */
> +static inline int arch_wants_pte_order(struct vm_area_struct *vma)
> +{
> +	return ilog2(SZ_64K >> PAGE_SHIFT);
Default value which is not related with any silicon may be: PAGE_ALLOC_COSTLY_ORDER?

Also, current pcp list support cache page with order 0...PAGE_ALLOC_COSTLY_ORDER, 9.
If the pcp could cover the page, the pressure to zone lock will be reduced by pcp.


Regards
Yin, Fengwei

> +}
> +#endif
> +
>  #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>  static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>  				       unsigned long address,