linux-kernel - Re: [PATCH v4 4/4] arm64: Add batched versions of ptep_modify_prot

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Message-ID: <051d5338-d073-4a92-abd2-c68367c17636@arm.com>
Date: Mon, 30 Jun 2025 11:43:23 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: david@...hat.com, willy@...radead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
 Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, vbabka@...e.cz,
 jannh@...gle.com, anshuman.khandual@....com, peterx@...hat.com,
 joey.gouly@....com, ioworker0@...il.com, baohua@...nel.org,
 kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
 christophe.leroy@...roup.eu, yangyicong@...ilicon.com,
 linux-arm-kernel@...ts.infradead.org, hughd@...gle.com,
 yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v4 4/4] arm64: Add batched versions of
 ptep_modify_prot_start/commit

On 28/06/2025 12:34, Dev Jain wrote:
> Override the generic definition of modify_prot_start_ptes() to use
> get_and_clear_full_ptes(). This helper does a TLBI only for the starting
> and ending contpte block of the range, whereas the current implementation
> will call ptep_get_and_clear() for every contpte block, thus doing a
> TLBI on every contpte block. Therefore, we have a performance win.
> 
> The arm64 definition of pte_accessible() allows us to batch in the
> errata specific case:
> 
> #define pte_accessible(mm, pte)	\
> 	(mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
> 
> All ptes are obviously present in the folio batch, and they are also valid.
> 
> Override the generic definition of modify_prot_commit_ptes() to simply
> use set_ptes() to map the new ptes into the pagetable.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> ---
>  arch/arm64/include/asm/pgtable.h | 10 ++++++++++
>  arch/arm64/mm/mmu.c              | 28 +++++++++++++++++++++++-----
>  2 files changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index ba63c8736666..abd2dee416b3 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1643,6 +1643,16 @@ extern void ptep_modify_prot_commit(struct vm_area_struct *vma,
>  				    unsigned long addr, pte_t *ptep,
>  				    pte_t old_pte, pte_t new_pte);
>  
> +#define modify_prot_start_ptes modify_prot_start_ptes
> +extern pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
> +				    unsigned long addr, pte_t *ptep,
> +				    unsigned int nr);
> +
> +#define modify_prot_commit_ptes modify_prot_commit_ptes
> +extern void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned long addr,
> +				    pte_t *ptep, pte_t old_pte, pte_t pte,
> +				    unsigned int nr);
> +
>  #ifdef CONFIG_ARM64_CONTPTE
>  
>  /*
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 3d5fb37424ab..38325616f467 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -26,6 +26,7 @@
>  #include <linux/set_memory.h>
>  #include <linux/kfence.h>
>  #include <linux/pkeys.h>
> +#include <linux/mm_inline.h>
>  
>  #include <asm/barrier.h>
>  #include <asm/cputype.h>
> @@ -1524,24 +1525,41 @@ static int __init prevent_bootmem_remove_init(void)
>  early_initcall(prevent_bootmem_remove_init);
>  #endif
>  
> -pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
> +pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
> +			     pte_t *ptep, unsigned int nr)
>  {
> +	pte_t pte = get_and_clear_full_ptes(vma->vm_mm, addr, ptep, nr, 0);
> +
>  	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_2645198)) {
>  		/*
>  		 * Break-before-make (BBM) is required for all user space mappings
>  		 * when the permission changes from executable to non-executable
>  		 * in cases where cpu is affected with errata #2645198.
>  		 */
> -		if (pte_user_exec(ptep_get(ptep)))
> -			return ptep_clear_flush(vma, addr, ptep);
> +		if (pte_accessible(vma->vm_mm, pte) && pte_user_exec(pte))
> +			__flush_tlb_range(vma, addr, nr * PAGE_SIZE,
> +					  PAGE_SIZE, true, 3);
>  	}
> -	return ptep_get_and_clear(vma->vm_mm, addr, ptep);
> +
> +	return pte;
> +}
> +
> +pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
> +{
> +	return modify_prot_start_ptes(vma, addr, ptep, 1);
> +}
> +
> +void modify_prot_commit_ptes(struct vm_area_struct *vma, unsigned long addr,
> +			     pte_t *ptep, pte_t old_pte, pte_t pte,
> +			     unsigned int nr)
> +{
> +	set_ptes(vma->vm_mm, addr, ptep, pte, nr);
>  }
>  
>  void ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep,
>  			     pte_t old_pte, pte_t pte)
>  {
> -	set_pte_at(vma->vm_mm, addr, ptep, pte);
> +	modify_prot_commit_ptes(vma, addr, ptep, old_pte, pte, 1);
>  }
>  
>  /*