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

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Date:   Thu, 3 Sep 2020 17:56:32 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....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 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()).

> 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

> + * 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()?

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>

-- 
Catalin