linux-kernel - Re: [PATCH v2 2/3] arm64: mm: Optimize range_split_to

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Message-ID: <abc692b2-9c1c-47f6-a8f7-5a1f26a98b83@os.amperecomputing.com>
Date: Thu, 6 Nov 2025 12:47:27 -0800
From: Yang Shi <yang@...amperecomputing.com>
To: Ryan Roberts <ryan.roberts@....com>, catalin.marinas@....com,
 will@...nel.org, david@...hat.com, ardb@...nel.org, dev.jain@....com,
 scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] arm64: mm: Optimize range_split_to_ptes()



On 11/6/25 8:09 AM, Ryan Roberts wrote:
> Enter lazy_mmu mode while splitting a range of memory to pte mappings.
> This causes barriers, which would otherwise be emitted after every pte
> (and pmd/pud) write, to be deferred until exiting lazy_mmu mode.
>
> For large systems, this is expected to significantly speed up fallback
> to pte-mapping the linear map for the case where the boot CPU has
> BBML2_NOABORT, but secondary CPUs do not. I haven't directly measured
> it, but this is equivalent to commit 1fcb7cea8a5f ("arm64: mm: Batch dsb
> and isb when populating pgtables").
>
> Note that for the path from arch_kfence_init_pool(), we may sleep while
> allocating memory inside the lazy_mmu mode. Sleeping is not allowed by
> generic code inside lazy_mmu, but we know that the arm64 implementation
> is sleep-safe. So this is ok and follows the same pattern already used
> by split_kernel_leaf_mapping().
>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

Reviewed-by: Yang Shi <yang@...amperecomputing.com>

Thanks,
Yang

> ---
>   arch/arm64/mm/mmu.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index a364ac2c9c61..652bb8c14035 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -832,8 +832,14 @@ static const struct mm_walk_ops split_to_ptes_ops = {
>   
>   static int range_split_to_ptes(unsigned long start, unsigned long end, gfp_t gfp)
>   {
> -	return walk_kernel_page_table_range_lockless(start, end,
> +	int ret;
> +
> +	arch_enter_lazy_mmu_mode();
> +	ret = walk_kernel_page_table_range_lockless(start, end,
>   					&split_to_ptes_ops, NULL, &gfp);
> +	arch_leave_lazy_mmu_mode();
> +
> +	return ret;
>   }
>   
>   static bool linear_map_requires_bbml2 __initdata;