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Message-ID: <8b4eba5d-545e-c2a4-e4cf-fd5dd6634265@arm.com>
Date:   Mon, 27 Jun 2022 13:05:00 +0530
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Subject: Re: [PATCH] mm: hugetlb: kill set_huge_swap_pte_at()
Content-Language: en-US
To:     Qi Zheng <zhengqi.arch@bytedance.com>, mike.kravetz@oracle.com,
        songmuchun@bytedance.com, akpm@linux-foundation.org,
        catalin.marinas@arm.com, will@kernel.org
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20220626145717.53572-1-zhengqi.arch@bytedance.com>
 <f0cfe169-44fa-5653-d454-149ef286d3bb@arm.com>
 <037fc8c3-9b71-cb83-8882-95d5459a494f@bytedance.com>
 <b46f5390-830d-08fd-0d6d-3fda7d31c36a@arm.com>
 <d68572c8-a12b-1e3a-09d4-7a971ff305d6@bytedance.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <d68572c8-a12b-1e3a-09d4-7a971ff305d6@bytedance.com>
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On 6/27/22 12:59, Qi Zheng wrote:
> 
> 
> On 2022/6/27 15:14, Anshuman Khandual wrote:
>>
>>
>> On 6/27/22 12:25, Qi Zheng wrote:
>>>
>>>
>>> On 2022/6/27 14:18, Anshuman Khandual wrote:
>>>>
>>>>
>>>> On 6/26/22 20:27, Qi Zheng wrote:
>>>>> The commit e5251fd43007 ("mm/hugetlb: introduce set_huge_swap_pte_at()
>>>>> helper") add set_huge_swap_pte_at() to handle swap entries on
>>>>> architectures that support hugepages consisting of contiguous ptes.
>>>>> And currently the set_huge_swap_pte_at() is only overridden by arm64.
>>>>>
>>>>> The set_huge_swap_pte_at() provide a sz parameter to help determine
>>>>> the number of entries to be updated. But in fact, all hugetlb swap
>>>>> entries contain pfn information, so we can find the corresponding
>>>>> folio through the pfn recorded in the swap entry, then the folio_size()
>>>>> is the number of entries that need to be updated.
>>>>>
>>>>> And considering that users will easily cause bugs by ignoring the
>>>>> difference between set_huge_swap_pte_at() and set_huge_pte_at().
>>>>> Let's handle swap entries in set_huge_pte_at() and remove the
>>>>> set_huge_swap_pte_at(), then we can call set_huge_pte_at()
>>>>> anywhere, which simplifies our coding.
>>>>>
>>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>>> ---
>>>>>    arch/arm64/include/asm/hugetlb.h |  3 ---
>>>>>    arch/arm64/mm/hugetlbpage.c      | 34 ++++++++++++++++----------------
>>>>>    include/linux/hugetlb.h          | 13 ------------
>>>>>    mm/hugetlb.c                     |  8 +++-----
>>>>>    mm/rmap.c                        | 11 +++--------
>>>>>    5 files changed, 23 insertions(+), 46 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h
>>>>> index 1fd2846dbefe..d20f5da2d76f 100644
>>>>> --- a/arch/arm64/include/asm/hugetlb.h
>>>>> +++ b/arch/arm64/include/asm/hugetlb.h
>>>>> @@ -46,9 +46,6 @@ extern void huge_pte_clear(struct mm_struct *mm, unsigned long addr,
>>>>>                   pte_t *ptep, unsigned long sz);
>>>>>    #define __HAVE_ARCH_HUGE_PTEP_GET
>>>>>    extern pte_t huge_ptep_get(pte_t *ptep);
>>>>> -extern void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr,
>>>>> -                 pte_t *ptep, pte_t pte, unsigned long sz);
>>>>> -#define set_huge_swap_pte_at set_huge_swap_pte_at
>>>>>      void __init arm64_hugetlb_cma_reserve(void);
>>>>>    diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
>>>>> index c9e076683e5d..58b89b9d13e0 100644
>>>>> --- a/arch/arm64/mm/hugetlbpage.c
>>>>> +++ b/arch/arm64/mm/hugetlbpage.c
>>>>> @@ -238,6 +238,13 @@ static void clear_flush(struct mm_struct *mm,
>>>>>        flush_tlb_range(&vma, saddr, addr);
>>>>>    }
>>>>>    +static inline struct folio *hugetlb_swap_entry_to_folio(swp_entry_t entry)
>>>>> +{
>>>>> +    VM_BUG_ON(!is_migration_entry(entry) && !is_hwpoison_entry(entry));
>>>>> +
>>>>> +    return page_folio(pfn_to_page(swp_offset(entry)));
>>>>> +}
>>>>
>>>> Extracting this huge page size from swap entry is an additional operation which
>>>> will increase the over all cost for set_huge_swap_pte_at(). At present the size
>>>
>>> Hmm, I think this cost is very small. And replacing
>>> set_huge_swap_pte_at() by transparently handling swap entries helps
>>> reduce possible bugs, which is worthwhile.
>>
>> Possible bugs ? There are just six call sites for this function.
> 
> For the swap entry of hugetlb, we need to remember that we should
> call set_huge_swap_pte_at() instead of set_huge_pte_at() every

Which is intuitive, so what is the problem ?

> time. And the difference between the two is only on arm64, other
> architectures should not need to pay attention to this.

Hence there is a generic fallback for other platforms.

> 
>> Although this proposed patch is functionally correct, I dont see
>> a valid enough reason to increase the overall cost in the path.
>