linux-kernel - Re: [PATCH v9 10/21] mm: Add generic p?d

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Date:   Tue, 23 Jul 2019 10:41:14 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Steven Price <steven.price@....com>
Cc:     linux-mm@...ck.org, Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v9 10/21] mm: Add generic p?d_leaf() macros

On Mon, Jul 22, 2019 at 04:41:59PM +0100, Steven Price wrote:
> Exposing the pud/pgd levels of the page tables to walk_page_range() means
> we may come across the exotic large mappings that come with large areas
> of contiguous memory (such as the kernel's linear map).
> 
> For architectures that don't provide all p?d_leaf() macros, provide
> generic do nothing default that are suitable where there cannot be leaf
> pages that that level.
> 
> Signed-off-by: Steven Price <steven.price@....com>

Not a big deal, but it would probably make sense for this to be patch 1
in the series, given it defines the semantic of p?d_leaf(), and they're
not used until we provide all the architectural implemetnations anyway.

It might also be worth pointing out the reasons for this naming, e.g.
p?d_large() aren't currently generic, and this name minimizes potential
confusion between p?d_{large,huge}().

> ---
>  include/asm-generic/pgtable.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index 75d9d68a6de7..46275896ca66 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -1188,4 +1188,23 @@ static inline bool arch_has_pfn_modify_check(void)
>  #define mm_pmd_folded(mm)	__is_defined(__PAGETABLE_PMD_FOLDED)
>  #endif
>  
> +/*
> + * p?d_leaf() - true if this entry is a final mapping to a physical address.
> + * This differs from p?d_huge() by the fact that they are always available (if
> + * the architecture supports large pages at the appropriate level) even
> + * if CONFIG_HUGETLB_PAGE is not defined.
> + */

I assume it's only safe to call these on valid entries? I think it would
be worth calling that out explicitly.

Otherwise, this looks sound to me:

Acked-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.

> +#ifndef pgd_leaf
> +#define pgd_leaf(x)	0
> +#endif
> +#ifndef p4d_leaf
> +#define p4d_leaf(x)	0
> +#endif
> +#ifndef pud_leaf
> +#define pud_leaf(x)	0
> +#endif
> +#ifndef pmd_leaf
> +#define pmd_leaf(x)	0
> +#endif
> +
>  #endif /* _ASM_GENERIC_PGTABLE_H */
> -- 
> 2.20.1
>