linux-kernel - Re: [PATCH v2 4/6] arm64/mm: Implement pXX_leaf

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Message-ID: <20201126143249.GA18344@willie-the-truck>
Date:   Thu, 26 Nov 2020 14:32:50 +0000
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kan.liang@...ux.intel.com, mingo@...nel.org, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, eranian@...gle.com, christophe.leroy@...roup.eu,
        npiggin@...il.com, linuxppc-dev@...ts.ozlabs.org,
        mpe@...erman.id.au, willy@...radead.org,
        aneesh.kumar@...ux.ibm.com, sparclinux@...r.kernel.org,
        davem@...emloft.net, catalin.marinas@....com,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, dave.hansen@...el.com,
        kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v2 4/6] arm64/mm: Implement pXX_leaf_size() support

On Thu, Nov 26, 2020 at 01:57:47PM +0100, Peter Zijlstra wrote:
> 
> Now with pmd_cont() defined...
> 
> ---
> Subject: arm64/mm: Implement pXX_leaf_size() support
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Fri Nov 13 11:46:06 CET 2020
> 
> ARM64 has non-pagetable aligned large page support with PTE_CONT, when
> this bit is set the page is part of a super-page. Match the hugetlb
> code and support these super pages for PTE and PMD levels.
> 
> This enables PERF_SAMPLE_{DATA,CODE}_PAGE_SIZE to report accurate
> pagetable leaf sizes.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/arm64/include/asm/pgtable.h |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -407,6 +407,7 @@ static inline int pmd_trans_huge(pmd_t p
>  #define pmd_dirty(pmd)		pte_dirty(pmd_pte(pmd))
>  #define pmd_young(pmd)		pte_young(pmd_pte(pmd))
>  #define pmd_valid(pmd)		pte_valid(pmd_pte(pmd))
> +#define pmd_cont(pmd)		pte_cont(pmd_pte(pmd))
>  #define pmd_wrprotect(pmd)	pte_pmd(pte_wrprotect(pmd_pte(pmd)))
>  #define pmd_mkold(pmd)		pte_pmd(pte_mkold(pmd_pte(pmd)))
>  #define pmd_mkwrite(pmd)	pte_pmd(pte_mkwrite(pmd_pte(pmd)))
> @@ -503,6 +504,9 @@ extern pgprot_t phys_mem_access_prot(str
>  				 PMD_TYPE_SECT)
>  #define pmd_leaf(pmd)		pmd_sect(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)
> +
>  #if defined(CONFIG_ARM64_64K_PAGES) || CONFIG_PGTABLE_LEVELS < 3
>  static inline bool pud_sect(pud_t pud) { return false; }
>  static inline bool pud_table(pud_t pud) { return true; }

Acked-by: Will Deacon <will@...nel.org>

I'm still highly dubious about the utility of this feature in perf, since
the TLB entry size is pretty much independent of the page-table
configuration, but that's a problem for all architectures I suspect.

Will