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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/30/25 11:19, Dev Jain wrote:
> 
> 
> 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.

But is not splitting the TLBI flush responsibility between the callers
(intermediate blocks) and the platform API (contpte-edges) - some what
problematic from a semantics perspective, and will be more susceptible
for missing TLB flushes etc ?

> 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
>