linux-kernel - Re: [PATCH 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics

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Message-ID: <b29e2bda-3299-e69f-90cf-1c9bd59a219d@arm.com>
Date:   Tue, 8 Sep 2020 15:48:08 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        will@...nel.org, akpm@...ux-foundation.org,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Suzuki Poulose <suzuki.poulose@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64/mm: Change THP helpers to comply with generic
 MM semantics



On 09/03/2020 10:26 PM, Catalin Marinas wrote:
> On Mon, Aug 17, 2020 at 02:49:43PM +0530, Anshuman Khandual wrote:
>> pmd_present() and pmd_trans_huge() are expected to behave in the following
>> manner during various phases of a given PMD. It is derived from a previous
>> detailed discussion on this topic [1] and present THP documentation [2].
>>
>> pmd_present(pmd):
>>
>> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
>> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(pmd)
> 
> The second bullet doesn't make much sense. If you have a pmd mapping of
> some I/O memory, pmd_present() still returns true (as does
> pte_present()).

Derived this from an earlier discussion (https://lkml.org/lkml/2018/10/17/231)
but current representation here might not be accurate.

Would this be any better ?

pmd_present(pmd):

- Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
- Returns false if pmd refers to a migration or swap entry

> 
>> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
>> index 4d867c6446c4..28792fdd9627 100644
>> --- a/arch/arm64/include/asm/pgtable-prot.h
>> +++ b/arch/arm64/include/asm/pgtable-prot.h
>> @@ -19,6 +19,13 @@
>>  #define PTE_DEVMAP		(_AT(pteval_t, 1) << 57)
>>  #define PTE_PROT_NONE		(_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */
>>  
>> +/*
>> + * This help indicate that the entry is present i.e pmd_page()
> 
> Nit: add another . after i.e

Will fix.

> 
>> + * still points to a valid huge page in memory even if the pmd
>> + * has been invalidated.
>> + */
>> +#define PMD_PRESENT_INVALID	(_AT(pteval_t, 1) << 59) /* only when !PMD_SECT_VALID */
>> +
>>  #ifndef __ASSEMBLY__
>>  
>>  #include <asm/cpufeature.h>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index d5d3fbe73953..7aa69cace784 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -145,6 +145,18 @@ static inline pte_t set_pte_bit(pte_t pte, pgprot_t prot)
>>  	return pte;
>>  }
>>  
>> +static inline pmd_t clr_pmd_bit(pmd_t pmd, pgprot_t prot)
>> +{
>> +	pmd_val(pmd) &= ~pgprot_val(prot);
>> +	return pmd;
>> +}
> 
> Could you use clear_pmd_bit (instead of clr) for consistency with
> clear_pte_bit()?

Sure, will do.

> 
> It would be good if the mm folk can do a sanity check on the assumptions
> about pmd_present/pmdp_invalidate/pmd_trans_huge.
> 
> The patch looks fine to me otherwise, feel free to add:
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
>