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Subject: Re: [PATCH] LoongArch: let pmd_present return true when splitting pmd
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     Huacai Chen <chenhuacai@kernel.org>,
        WANG Xuerui <kernel@xen0n.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        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>,
        loongarch@lists.linux.dev, linux-kernel@vger.kernel.org,
        loongson-kernel@lists.loongnix.cn
References: <20230524074132.10916-1-zhanghongchen@loongson.cn>
 <CAAhV-H4CC0S7ciLCqdb_AoQ+3-24uo6TZb9Pz7H1yu+mvspFDw@mail.gmail.com>
From:   Hongchen Zhang <zhanghongchen@loongson.cn>
Message-ID: <a8479383-a0d4-0c0a-20ef-d3c64d9ea192@loongson.cn>
Date:   Thu, 8 Jun 2023 10:12:25 +0800
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Hi Anshuman,

On 2023/5/27 下午1:32, Huacai Chen wrote:
> Hi, Anshuman,
> 
> Excuse me, but could you please tell me something that why commit
> b65399f6111b ("arm64/mm: Change THP helpers to comply with generic MM
> semantics") needn't be backported to 5.4LTS and 4.19LTS series. The
> discussion link you referred is as early as in 2018, before 4.19
> released. I think this information is important for us because we want
> to know how to handle our 4.19-loongarch codebase.

What's your opinion about huacai's question? In my opinion,your commit
should be backported to 5.4LTS and 4.19LTS.

> 
> Huacai
> 
> On Wed, May 24, 2023 at 3:42 PM Hongchen Zhang
> <zhanghongchen@loongson.cn> 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
>> --
>> 2.31.1
>>
>>

Best Regards
Hongchen Zhang