linux-kernel - Re: [PATCH v3 2/4] mm: Default implementation of arch_wants_pte

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Message-ID: <86c11e91-5dc5-a185-59a1-afa1e3295060@intel.com>
Date:   Mon, 17 Jul 2023 19:13:27 +0800
From:   Yin Fengwei <fengwei.yin@...el.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH v3 2/4] mm: Default implementation of
 arch_wants_pte_order()



On 7/15/23 00:17, 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 -1, implying that the HW
> has no preference. In this case, mm will choose it's own default order.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>


Regards
Yin, Fengwei

> ---
>  include/linux/pgtable.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 5063b482e34f..2a1d83775837 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -313,6 +313,19 @@ 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. Negative value implies that the HW has no preference
> + * and mm will choose it's own default order.
> + */
> +static inline int arch_wants_pte_order(void)
> +{
> +	return -1;
> +}
> +#endif
> +
>  #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
>  static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>  				       unsigned long address,