Re: [PATCH v2 4/7] arm64: Add batched version of ptep_modify_prot_start

[Dev Jain <dev.jain@arm.com>]

On 30/04/25 11:13 am, Anshuman Khandual wrote:
> On 4/29/25 10:53, Dev Jain wrote:
>> Override the generic definition to use get_and_clear_full_ptes(), so that
>> we do a TLBI possibly only on the "contpte-edges" of the large PTE block,
>> instead of doing it for every contpte block, which happens for ptep_get_and_clear().
> 
> Could you please explain what does "contpte-edges" really signify in the
> context of large PTE blocks ? Also how TLBI operation only on these edges
> will never run into the risk of missing TLB invalidation of some other
> mapped areas ?

We are doing a TLBI over the whole range already, in the mprotect code:
see tlb_flush_pte_range. What the arm64 internal API does, irrespective 
of the caller, is to do a TLBI for every contpte block in case of 
unfolding. We don't need that for the intermediate blocks because the 
caller does that. We do need a TLBI for the start and end contpte block,
because in case the range we are invalidating partially covers them, 
then the caller will not do a TLBI for the non-overlapped PTEs of the block.
I'll explain some more in the changelog next version.

> 
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   arch/arm64/include/asm/pgtable.h |  5 +++++
>>   arch/arm64/mm/mmu.c              | 12 +++++++++---
>>   include/linux/pgtable.h          |  4 ++++
>>   mm/pgtable-generic.c             | 16 +++++++++++-----
>>   4 files changed, 29 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 2a77f11b78d5..8872ea5f0642 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -1553,6 +1553,11 @@ 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);
>> +
>>   #ifdef CONFIG_ARM64_CONTPTE
>>   
>>   /*
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 8fcf59ba39db..fe60be8774f4 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1523,7 +1523,8 @@ 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)
>>   {
>>   	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_2645198)) {
>>   		/*
>> @@ -1532,9 +1533,14 @@ pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte
>>   		 * in cases where cpu is affected with errata #2645198.
>>   		 */
>>   		if (pte_user_exec(ptep_get(ptep)))
>> -			return ptep_clear_flush(vma, addr, ptep);
>> +			return clear_flush_ptes(vma, addr, ptep, nr);
>>   	}
>> -	return ptep_get_and_clear(vma->vm_mm, addr, ptep);
>> +	return get_and_clear_full_ptes(vma->vm_mm, addr, ptep, nr, 0);
>> +}
>> +
>> +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 ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep,
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index ed287289335f..10cdb87ccecf 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -828,6 +828,10 @@ extern pte_t ptep_clear_flush(struct vm_area_struct *vma,
>>   			      pte_t *ptep);
>>   #endif
>>   
>> +extern pte_t clear_flush_ptes(struct vm_area_struct *vma,
>> +			      unsigned long address,
>> +			      pte_t *ptep, unsigned int nr);
>> +
>>   #ifndef __HAVE_ARCH_PMDP_HUGE_CLEAR_FLUSH
>>   extern pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma,
>>   			      unsigned long address,
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 5a882f2b10f9..e238f88c3cac 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -90,17 +90,23 @@ int ptep_clear_flush_young(struct vm_area_struct *vma,
>>   }
>>   #endif
>>   
>> -#ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
>> -pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long address,
>> -		       pte_t *ptep)
>> +pte_t clear_flush_ptes(struct vm_area_struct *vma, unsigned long address,
>> +		       pte_t *ptep, unsigned int nr)
>>   {
>>   	struct mm_struct *mm = (vma)->vm_mm;
>>   	pte_t pte;
>> -	pte = ptep_get_and_clear(mm, address, ptep);
>> +	pte = get_and_clear_full_ptes(mm, address, ptep, nr, 0);
>>   	if (pte_accessible(mm, pte))
>> -		flush_tlb_page(vma, address);
>> +		flush_tlb_range(vma, address, address + nr * PAGE_SIZE);
>>   	return pte;
>>   }
>> +
>> +#ifndef __HAVE_ARCH_PTEP_CLEAR_FLUSH
>> +pte_t ptep_clear_flush(struct vm_area_struct *vma, unsigned long address,
>> +		       pte_t *ptep)
>> +{
>> +	return clear_flush_ptes(vma, address, ptep, 1);
>> +}
>>   #endif
>>   
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE