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

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Message-ID: <7e7145b0-149c-c09d-08a6-99227519df87@arm.com>
Date:   Mon, 17 Jul 2023 14:15:18 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yin Fengwei <fengwei.yin@...el.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 17/07/2023 14:01, David Hildenbrand wrote:
> 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@....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.

There was a lot more in this at v1 IIRC, so made more sense as standalone. I
agree it can be squashed into the next patch now. Will do for next version.

>