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Date:   Mon, 17 Jul 2023 15:01:23 +0200
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Subject: Re: [PATCH v3 2/4] mm: Default implementation of
 arch_wants_pte_order()
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To:     Ryan Roberts <ryan.roberts@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
        Luis Chamberlain <mcgrof@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20230714160407.4142030-1-ryan.roberts@arm.com>
 <20230714161733.4144503-2-ryan.roberts@arm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230714161733.4144503-2-ryan.roberts@arm.com>
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On 14.07.23 18:17, 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@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 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,

What is the reason to have this into a separate patch? That should 
simply be squashed into the actual user -- patch #3.

-- 
Cheers,

David / dhildenb