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

Catalin Marinas
2020-09-03T16:56:39Z

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@arm.com>

--
Catalin