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Subject: Re: Re: [PATCH] LoongArch: let pmd_present return true when splitting
 pmd
To:     "bibo, mao" <maobibo@loongson.cn>,
        Huacai Chen <chenhuacai@kernel.org>,
        WANG Xuerui <kernel@xen0n.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Hildenbrand <david@redhat.com>,
        Mike Rapoport <rppt@kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Feiyang Chen <chenfeiyang@loongson.cn>,
        Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     loongson-kernel@lists.loongnix.cn, loongarch@lists.linux.dev,
        linux-kernel@vger.kernel.org
References: <20230524074132.10916-1-zhanghongchen@loongson.cn>
 <46ce41a3-16bd-6d92-c8a8-764283859190@loongson.cn>
 <fbe8a1ec-cc81-d4db-8c55-31a60fb9fb58@loongson.cn>
From:   Hongchen Zhang <zhanghongchen@loongson.cn>
Message-ID: <c628c83d-424d-5c0e-24ca-702252779940@loongson.cn>
Date:   Thu, 8 Jun 2023 09:31:24 +0800
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Hi Bibo & Anshuman,

On 2023/6/6 下午9:49, bibo, mao wrote:
> I do not object to adding sw bit for _PAGE_PRESENT_INVALID,  only that can it use
> bit12--bit20 since it is for pmd entry only?
If we change from bit60 to bit12, pmd_pfn will return an incorrect pfn, 
and it seems that modifying pmd_pfn accordingly is not a good idea.

Hi Anshuman,
   What's your opinion about this suggestion? I think arm64 architecture 
  has similar problems. Why do you choose bit59 instead of bit12--bit20?
> 
> Regards
> Bibo, Mao
> 
> 在 2023/6/6 19:38, Hongchen Zhang 写道:
>> Hi,
>>
>> Gentle ping.
>>
>> On 2023/5/24 pm 3:41, Hongchen Zhang wrote:
>>> when we split a pmd into ptes, pmd_present() and pmd_trans_huge() should
>>> return true,otherwise it would be treated as a swap pmd.
>>> As arm64 does in
>>> commit b65399f6111b ("arm64/mm: Change THP helpers to comply with generic MM semantics")
>>> we add a _PAGE_PRESENT_INVALID bit for LoongArch.
>>>
>>> Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>
>>> ---
>>>    arch/loongarch/include/asm/pgtable-bits.h | 2 ++
>>>    arch/loongarch/include/asm/pgtable.h      | 3 ++-
>>>    2 files changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/loongarch/include/asm/pgtable-bits.h b/arch/loongarch/include/asm/pgtable-bits.h
>>> index 8b98d22a145b..a7469d28d9d0 100644
>>> --- a/arch/loongarch/include/asm/pgtable-bits.h
>>> +++ b/arch/loongarch/include/asm/pgtable-bits.h
>>> @@ -22,12 +22,14 @@
>>>    #define    _PAGE_PFN_SHIFT        12
>>>    #define    _PAGE_SWP_EXCLUSIVE_SHIFT 23
>>>    #define    _PAGE_PFN_END_SHIFT    48
>>> +#define _PAGE_PRESENT_INVALID_SHIFT 60
>>>    #define    _PAGE_NO_READ_SHIFT    61
>>>    #define    _PAGE_NO_EXEC_SHIFT    62
>>>    #define    _PAGE_RPLV_SHIFT    63
>>>      /* Used by software */
>>>    #define _PAGE_PRESENT        (_ULCAST_(1) << _PAGE_PRESENT_SHIFT)
>>> +#define _PAGE_PRESENT_INVALID    (_ULCAST_(1) << _PAGE_PRESENT_INVALID_SHIFT)
>>>    #define _PAGE_WRITE        (_ULCAST_(1) << _PAGE_WRITE_SHIFT)
>>>    #define _PAGE_ACCESSED        (_ULCAST_(1) << _PAGE_ACCESSED_SHIFT)
>>>    #define _PAGE_MODIFIED        (_ULCAST_(1) << _PAGE_MODIFIED_SHIFT)
>>> diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
>>> index d28fb9dbec59..9a9f9ff9b709 100644
>>> --- a/arch/loongarch/include/asm/pgtable.h
>>> +++ b/arch/loongarch/include/asm/pgtable.h
>>> @@ -213,7 +213,7 @@ static inline int pmd_bad(pmd_t pmd)
>>>    static inline int pmd_present(pmd_t pmd)
>>>    {
>>>        if (unlikely(pmd_val(pmd) & _PAGE_HUGE))
>>> -        return !!(pmd_val(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE));
>>> +        return !!(pmd_val(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PRESENT_INVALID));
>>>          return pmd_val(pmd) != (unsigned long)invalid_pte_table;
>>>    }
>>> @@ -558,6 +558,7 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
>>>      static inline pmd_t pmd_mkinvalid(pmd_t pmd)
>>>    {
>>> +    pmd_val(pmd) |= _PAGE_PRESENT_INVALID;
>>>        pmd_val(pmd) &= ~(_PAGE_PRESENT | _PAGE_VALID | _PAGE_DIRTY | _PAGE_PROTNONE);
>>>          return pmd;
>>>
>>> base-commit: f1fcbaa18b28dec10281551dfe6ed3a3ed80e3d6
>>>
>>
>> Best Regards
>> Hongchen Zhang
>>
>> _______________________________________________
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> 
>