linux-kernel - Re: [PATCH v1 12/16] arm64/mm: Support huge pte-mapped pages in vmap

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Message-ID: <9a0d3009-18fc-4b53-941a-b6d830fce36a@arm.com>
Date: Fri, 7 Feb 2025 15:34:27 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Dev Jain <dev.jain@....com>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Steve Capper <steve.capper@...aro.org>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 12/16] arm64/mm: Support huge pte-mapped pages in vmap



On 2/5/25 20:39, Ryan Roberts wrote:
> Implement the required arch functions to enable use of contpte in the
> vmap when VM_ALLOW_HUGE_VMAP is specified. This speeds up vmap
> operations due to only having to issue a DSB and ISB per contpte block
> instead of per pte. But it also means that the TLB pressure reduces due
> to only needing a single TLB entry for the whole contpte block.

Right.

> 
> Since vmap uses set_huge_pte_at() to set the contpte, that API is now
> used for kernel mappings for the first time. Although in the vmap case
> we never expect it to be called to modify a valid mapping so
> clear_flush() should never be called, it's still wise to make it robust
> for the kernel case, so amend the tlb flush function if the mm is for
> kernel space.

Makes sense.

> 
> Tested with vmalloc performance selftests:
> 
>   # kself/mm/test_vmalloc.sh \
> 	run_test_mask=1
> 	test_repeat_count=5
> 	nr_pages=256
> 	test_loop_count=100000
> 	use_huge=1
> 
> Duration reduced from 1274243 usec to 1083553 usec on Apple M2 for 15%
> reduction in time taken.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>  arch/arm64/include/asm/vmalloc.h | 40 ++++++++++++++++++++++++++++++++
>  arch/arm64/mm/hugetlbpage.c      |  5 +++-
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h
> index 38fafffe699f..fbdeb40f3857 100644
> --- a/arch/arm64/include/asm/vmalloc.h
> +++ b/arch/arm64/include/asm/vmalloc.h
> @@ -23,6 +23,46 @@ static inline bool arch_vmap_pmd_supported(pgprot_t prot)
>  	return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS);
>  }
>  
> +#define arch_vmap_pte_range_map_size arch_vmap_pte_range_map_size
> +static inline unsigned long arch_vmap_pte_range_map_size(unsigned long addr,
> +						unsigned long end, u64 pfn,
> +						unsigned int max_page_shift)
> +{
> +	if (max_page_shift < CONT_PTE_SHIFT)
> +		return PAGE_SIZE;
> +
> +	if (end - addr < CONT_PTE_SIZE)
> +		return PAGE_SIZE;
> +
> +	if (!IS_ALIGNED(addr, CONT_PTE_SIZE))
> +		return PAGE_SIZE;
> +
> +	if (!IS_ALIGNED(PFN_PHYS(pfn), CONT_PTE_SIZE))
> +		return PAGE_SIZE;
> +
> +	return CONT_PTE_SIZE;

A small nit:

Should the rationale behind picking CONT_PTE_SIZE be added here as an in code
comment or something in the function - just to make things bit clear.

> +}
> +
> +#define arch_vmap_pte_range_unmap_size arch_vmap_pte_range_unmap_size
> +static inline unsigned long arch_vmap_pte_range_unmap_size(unsigned long addr,
> +							   pte_t *ptep)
> +{
> +	/*
> +	 * The caller handles alignment so it's sufficient just to check
> +	 * PTE_CONT.
> +	 */
> +	return pte_valid_cont(__ptep_get(ptep)) ? CONT_PTE_SIZE : PAGE_SIZE;

I guess it is safe to query the CONT_PTE from the mapped entry itself.

> +}
> +
> +#define arch_vmap_pte_supported_shift arch_vmap_pte_supported_shift
> +static inline int arch_vmap_pte_supported_shift(unsigned long size)
> +{
> +	if (size >= CONT_PTE_SIZE)
> +		return CONT_PTE_SHIFT;
> +
> +	return PAGE_SHIFT;
> +}
> +
>  #endif
>  
>  #define arch_vmap_pgprot_tagged arch_vmap_pgprot_tagged
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 02afee31444e..a74e43101dad 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -217,7 +217,10 @@ static void clear_flush(struct mm_struct *mm,
>  	for (i = 0; i < ncontig; i++, addr += pgsize, ptep++)
>  		___ptep_get_and_clear(mm, ptep, pgsize);
>  
> -	__flush_hugetlb_tlb_range(&vma, saddr, addr, pgsize, true);
> +	if (mm == &init_mm)
> +		flush_tlb_kernel_range(saddr, addr);
> +	else
> +		__flush_hugetlb_tlb_range(&vma, saddr, addr, pgsize, true);
>  }
>  
>  void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,

Otherwise LGTM.

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>