linux-kernel - Re: [PATCH v7 6/6] arm64: mm: Optimize linear_map_split_to

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Message-ID: <9d666caf-a799-4229-86b5-3a22a60fa79f@os.amperecomputing.com>
Date: Fri, 29 Aug 2025 15:27:31 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Ard Biesheuvel <ardb@...nel.org>, Dev Jain <dev.jain@....com>,
 scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v7 6/6] arm64: mm: Optimize linear_map_split_to_ptes()



On 8/29/25 4:52 AM, Ryan Roberts wrote:
> When splitting kernel leaf mappings, either via
> split_kernel_leaf_mapping_locked() or linear_map_split_to_ptes(),
> previously a leaf mapping was always split to the next size down. e.g.
> pud -> contpmd -> pmd -> contpte -> pte. But for
> linear_map_split_to_ptes() we can avoid the contpmd and contpte states
> because we know we want to split all the way down to ptes.
>
> This avoids visiting all the ptes in a table if it was created by
> splitting a pmd, which is noticible on systems with a lot of memory.

Similar to patch #4, this patch should be squashed into patch #5 IMHO.

Thanks,
Yang

>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>   arch/arm64/mm/mmu.c | 26 ++++++++++++++++++--------
>   1 file changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 6bd0b065bd97..8e45cd08bf3a 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -550,7 +550,7 @@ static void split_contpte(pte_t *ptep)
>   		__set_pte(ptep, pte_mknoncont(__ptep_get(ptep)));
>   }
>   
> -static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp)
> +static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp, bool to_cont)
>   {
>   	pmdval_t tableprot = PMD_TYPE_TABLE | PMD_TABLE_UXN | PMD_TABLE_AF;
>   	unsigned long pfn = pmd_pfn(pmd);
> @@ -568,7 +568,9 @@ static int split_pmd(pmd_t *pmdp, pmd_t pmd, gfp_t gfp)
>   		tableprot |= PMD_TABLE_PXN;
>   
>   	prot = __pgprot((pgprot_val(prot) & ~PTE_TYPE_MASK) | PTE_TYPE_PAGE);
> -	prot = __pgprot(pgprot_val(prot) | PTE_CONT);
> +	prot = __pgprot(pgprot_val(prot) & ~PTE_CONT);
> +	if (to_cont)
> +		prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>   
>   	for (i = 0; i < PTRS_PER_PTE; i++, ptep++, pfn++)
>   		__set_pte(ptep, pfn_pte(pfn, prot));
> @@ -592,7 +594,7 @@ static void split_contpmd(pmd_t *pmdp)
>   		set_pmd(pmdp, pmd_mknoncont(pmdp_get(pmdp)));
>   }
>   
> -static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp)
> +static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp, bool to_cont)
>   {
>   	pudval_t tableprot = PUD_TYPE_TABLE | PUD_TABLE_UXN | PUD_TABLE_AF;
>   	unsigned int step = PMD_SIZE >> PAGE_SHIFT;
> @@ -611,7 +613,9 @@ static int split_pud(pud_t *pudp, pud_t pud, gfp_t gfp)
>   		tableprot |= PUD_TABLE_PXN;
>   
>   	prot = __pgprot((pgprot_val(prot) & ~PMD_TYPE_MASK) | PMD_TYPE_SECT);
> -	prot = __pgprot(pgprot_val(prot) | PTE_CONT);
> +	prot = __pgprot(pgprot_val(prot) & ~PTE_CONT);
> +	if (to_cont)
> +		prot = __pgprot(pgprot_val(prot) | PTE_CONT);
>   
>   	for (i = 0; i < PTRS_PER_PMD; i++, pmdp++, pfn += step)
>   		set_pmd(pmdp, pfn_pmd(pfn, prot));
> @@ -669,7 +673,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>   	if (!pud_present(pud))
>   		goto out;
>   	if (pud_leaf(pud)) {
> -		ret = split_pud(pudp, pud, GFP_PGTABLE_KERNEL);
> +		ret = split_pud(pudp, pud, GFP_PGTABLE_KERNEL, true);
>   		if (ret)
>   			goto out;
>   	}
> @@ -694,7 +698,7 @@ static int split_kernel_leaf_mapping_locked(unsigned long addr)
>   		 */
>   		if (ALIGN_DOWN(addr, PMD_SIZE) == addr)
>   			goto out;
> -		ret = split_pmd(pmdp, pmd, GFP_PGTABLE_KERNEL);
> +		ret = split_pmd(pmdp, pmd, GFP_PGTABLE_KERNEL, true);
>   		if (ret)
>   			goto out;
>   	}
> @@ -771,7 +775,7 @@ static int __init split_to_ptes_pud_entry(pud_t *pudp, unsigned long addr,
>   	int ret = 0;
>   
>   	if (pud_leaf(pud))
> -		ret = split_pud(pudp, pud, GFP_ATOMIC);
> +		ret = split_pud(pudp, pud, GFP_ATOMIC, false);
>   
>   	return ret;
>   }
> @@ -786,7 +790,13 @@ static int __init split_to_ptes_pmd_entry(pmd_t *pmdp, unsigned long addr,
>   	if (pmd_leaf(pmd)) {
>   		if (pmd_cont(pmd))
>   			split_contpmd(pmdp);
> -		ret = split_pmd(pmdp, pmd, GFP_ATOMIC);
> +		ret = split_pmd(pmdp, pmd, GFP_ATOMIC, false);
> +
> +		/*
> +		 * We have split the pmd directly to ptes so there is no need to
> +		 * visit each pte to check if they are contpte.
> +		 */
> +		walk->action = ACTION_CONTINUE;
>   	}
>   
>   	return ret;