linux-kernel - Re: [PATCH] arm64/mm: Remove [PUD|PMD]_TABLE_BIT from [pud|pmd]

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Message-ID: <4a36d7b7-6b27-31cc-d701-ebe3c6e4946e@arm.com>
Date:   Tue, 11 May 2021 09:21:46 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Remove [PUD|PMD]_TABLE_BIT from [pud|pmd]_bad()


On 5/10/21 8:13 PM, Mark Rutland wrote:
> On Mon, May 10, 2021 at 04:37:51PM +0530, Anshuman Khandual wrote:
>> Semantics wise, [pud|pmd]_bad() have always implied that a given [PUD|PMD]
>> entry does not have a pointer to the next level page table. This had been
>> made clear in the commit a1c76574f345 ("arm64: mm: use *_sect to check for
>> section maps"). Hence explicitly check for a table entry rather than just
>> testing a single bit. This basically redefines [pud|pmd]_bad() in terms of
>> [pud|pmd]_table() making the semantics clear.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> 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>
> 
> I have no strong feelings either way, so: 
> 
> Acked-by: Mark Rutland <mark.rutland@....com>
> 
> ... that said, I think that the "bad" naming is unclear and misleading,
> and it'd be really nice if we could clean that up treewide with
> something clearer than "bad".

Agreed, the name is misleading.

> 
> It does seem that would roughly fit p??_leaf() if we had

But what if the platform does not support huge page aka leaf mapping
at the given level ? Also a non table i.e bad entry might not always
mean a leaf/section/huge page mapping, it could simply imply that the
entry is not just pointing to next level and might be just in an bad
intermediate or invalid state.

> p??_clear_leaf() and p??_none_or_clear_leaf() helpers.

Could you please elaborate how these new helpers might help define
pxx_bad() ?

> 
> Thanks,
> Mark.
> 
>> ---
>> This applies on v5.13-rc1.
>>
>>  arch/arm64/include/asm/pgtable.h | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 25f5c04b43ce..69f8183bef29 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -509,13 +509,12 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>>  
>>  #define pmd_none(pmd)		(!pmd_val(pmd))
>>  
>> -#define pmd_bad(pmd)		(!(pmd_val(pmd) & PMD_TABLE_BIT))
>> -
>>  #define pmd_table(pmd)		((pmd_val(pmd) & PMD_TYPE_MASK) == \
>>  				 PMD_TYPE_TABLE)
>>  #define pmd_sect(pmd)		((pmd_val(pmd) & PMD_TYPE_MASK) == \
>>  				 PMD_TYPE_SECT)
>>  #define pmd_leaf(pmd)		pmd_sect(pmd)
>> +#define pmd_bad(pmd)		(!pmd_table(pmd))
>>  
>>  #define pmd_leaf_size(pmd)	(pmd_cont(pmd) ? CONT_PMD_SIZE : PMD_SIZE)
>>  #define pte_leaf_size(pte)	(pte_cont(pte) ? CONT_PTE_SIZE : PAGE_SIZE)
>> @@ -602,7 +601,7 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
>>  	pr_err("%s:%d: bad pmd %016llx.\n", __FILE__, __LINE__, pmd_val(e))
>>  
>>  #define pud_none(pud)		(!pud_val(pud))
>> -#define pud_bad(pud)		(!(pud_val(pud) & PUD_TABLE_BIT))
>> +#define pud_bad(pud)		(!pud_table(pud))
>>  #define pud_present(pud)	pte_present(pud_pte(pud))
>>  #define pud_leaf(pud)		pud_sect(pud)
>>  #define pud_valid(pud)		pte_valid(pud_pte(pud))
>> -- 
>> 2.20.1
>>