linux-kernel - Re: [PATCH 1/5] arm64/mm: Drop pte

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Message-ID: <a91df2f6-e0d6-4902-ae22-5df132f00737@arm.com>
Date: Mon, 14 Oct 2024 14:29:49 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>, linux-arm-kernel@...ts.infradead.org
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 James Morse <james.morse@....com>, Catalin Marinas
 <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, Mark Rutland <mark.rutland@....com>,
 kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] arm64/mm: Drop pte_mkhuge()



On 10/9/24 18:50, Ryan Roberts wrote:
> On 05/10/2024 13:38, Anshuman Khandual wrote:
>> Core HugeTLB defines arch_make_huge_pte() fallback definition, which calls
>> platform provided pte_mkhuge(). But if any platform already provides custom
>> arch_make_huge_pte(), then it does not need to provide pte_mkhuge(). arm64
>> defines arch_make_huge_pte(), but then also calls pte_mkhuge() internally.
>> This creates confusion as if both of these callbacks are being used in core
>> HugeTLB and required to be defined in the platform.
>>
>> This changes arch_make_huge_pte() to create block mapping directly and also
>> drops off now redundant helper pte_mkhuge(), making things clear. Also this
>> changes HugeTLB page creation from just clearing the PTE_TABLE_BIT (bit[1])
>> to actually setting bits[1:0] via PTE_TYPE_[MASK|SECT] instead.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Ard Biesheuvel <ardb@...nel.org>
>> Cc: Ryan Roberts <ryan.roberts@....com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  arch/arm64/include/asm/pgtable-hwdef.h | 1 +
>>  arch/arm64/include/asm/pgtable.h       | 5 -----
>>  arch/arm64/mm/hugetlbpage.c            | 2 +-
>>  3 files changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
>> index fd330c1db289..956a702cb532 100644
>> --- a/arch/arm64/include/asm/pgtable-hwdef.h
>> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
>> @@ -158,6 +158,7 @@
>>  #define PTE_VALID		(_AT(pteval_t, 1) << 0)
>>  #define PTE_TYPE_MASK		(_AT(pteval_t, 3) << 0)
>>  #define PTE_TYPE_PAGE		(_AT(pteval_t, 3) << 0)
>> +#define PTE_TYPE_SECT		(_AT(pteval_t, 1) << 0)
>>  #define PTE_TABLE_BIT		(_AT(pteval_t, 1) << 1)
>>  #define PTE_USER		(_AT(pteval_t, 1) << 6)		/* AP[1] */
>>  #define PTE_RDONLY		(_AT(pteval_t, 1) << 7)		/* AP[2] */
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index c329ea061dc9..fa4c32a9f572 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -438,11 +438,6 @@ static inline void __set_ptes(struct mm_struct *mm,
>>  	}
>>  }
>>  
>> -/*
>> - * Huge pte definitions.
>> - */
>> -#define pte_mkhuge(pte)		(__pte(pte_val(pte) & ~PTE_TABLE_BIT))
>> -
>>  /*
>>   * Hugetlb definitions.
>>   */
>> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
>> index 5f1e2103888b..5922c95630ad 100644
>> --- a/arch/arm64/mm/hugetlbpage.c
>> +++ b/arch/arm64/mm/hugetlbpage.c
>> @@ -361,7 +361,7 @@ pte_t arch_make_huge_pte(pte_t entry, unsigned int shift, vm_flags_t flags)
>>  {
>>  	size_t pagesize = 1UL << shift;
>>  
>> -	entry = pte_mkhuge(entry);
>> +	entry = __pte((pte_val(entry) & ~PTE_TYPE_MASK) | PTE_TYPE_SECT);
> 
> I think there may be an existing bug here; if pagesize == CONT_PTE_SIZE, then
> entry will be placed in the level 3 table. In this case, shouldn't bit 1 remain
> set, because at level 3, a page mapping is denoted by bits[1:0] = 3 ? Currently
> its being unconditionally cleared.

That's not a problem, pte_mkcont() brings back both the bits
via PTE_TYPE_PAGE along with PTE_CONT.

        if (pagesize == CONT_PTE_SIZE) {
                entry = pte_mkcont(entry);
        } else if (pagesize == CONT_PMD_SIZE) {
                entry = pmd_pte(pmd_mkcont(pte_pmd(entry)));
        } else if (pagesize != PUD_SIZE && pagesize != PMD_SIZE) {
                pr_warn("%s: unrecognized huge page size 0x%lx\n",
                        __func__, pagesize);
        }

static inline pte_t pte_mkcont(pte_t pte)
{
	pte = set_pte_bit(pte, __pgprot(PTE_CONT));
      	return set_pte_bit(pte, __pgprot(PTE_TYPE_PAGE));
}

Although the same is not required for CONT_PMD_SIZE size huge
pages where only PTE_CONT is enough.

static inline pmd_t pmd_mkcont(pmd_t pmd)
{
        return __pmd(pmd_val(pmd) | PMD_SECT_CONT);
}
 
>>  	if (pagesize == CONT_PTE_SIZE) {
>>  		entry = pte_mkcont(entry);
>>  	} else if (pagesize == CONT_PMD_SIZE) {
>