Re: [PATCH v4 2/4] mm: Add batched versions of ptep_modify_prot_start/commit

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

On 30/06/25 3:40 pm, Ryan Roberts wrote:
> On 28/06/2025 12:34, Dev Jain wrote:
>> Batch ptep_modify_prot_start/commit in preparation for optimizing mprotect.
>> Architecture can override these helpers; in case not, they are implemented
>> as a simple loop over the corresponding single pte helpers.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   include/linux/pgtable.h | 83 ++++++++++++++++++++++++++++++++++++++++-
>>   mm/mprotect.c           |  4 +-
>>   2 files changed, 84 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index cf1515c163e2..662f39e7475a 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -1331,7 +1331,8 @@ static inline pte_t ptep_modify_prot_start(struct vm_area_struct *vma,
>>   
>>   /*
>>    * Commit an update to a pte, leaving any hardware-controlled bits in
>> - * the PTE unmodified.
>> + * the PTE unmodified. The pte may have been "upgraded" w.r.t a/d bits compared
>> + * to the old_pte, as in, it may have a/d bits on which were off in old_pte.
> I find this last sentance a bit confusing. I think what you are trying to say is
> somehthing like:
>
> """
> old_pte is the value returned from ptep_modify_prot_start() but may additionally
> have have young and/or dirty bits set where previously they were not.
> """

Thanks.

> ?
>
>>    */
>>   static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
>>   					   unsigned long addr,
>> @@ -1340,6 +1341,86 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
>>   	__ptep_modify_prot_commit(vma, addr, ptep, pte);
>>   }
>>   #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
>> +
>> +/**
>> + * modify_prot_start_ptes - Start a pte protection read-modify-write transaction
>> + * over a batch of ptes, which protects against asynchronous hardware
>> + * modifications to the ptes. The intention is not to prevent the hardware from
>> + * making pte updates, but to prevent any updates it may make from being lost.
>> + * Please see the comment above ptep_modify_prot_start() for full description.
>> + *
>> + * @vma: The virtual memory area the pages are mapped into.
>> + * @addr: Address the first page is mapped at.
>> + * @ptep: Page table pointer for the first entry.
>> + * @nr: Number of entries.
>> + *
>> + * May be overridden by the architecture; otherwise, implemented as a simple
>> + * loop over ptep_modify_prot_start(), collecting the a/d bits from each pte
>> + * in the batch.
>> + *
>> + * Note that PTE bits in the PTE batch besides the PFN can differ.
>> + *
>> + * Context: The caller holds the page table lock.  The PTEs map consecutive
>> + * pages that belong to the same folio.  The PTEs are all in the same PMD.
>> + * Since the batch is determined from folio_pte_batch, the PTEs must differ
>> + * only in a/d bits (and the soft dirty bit; see fpb_t flags in
>> + * mprotect_folio_pte_batch()).
> This last sentence is confusing... You had previous said the PFN can differ, but
> here you imply on a, d and sd bits are allowed to differ.

Forgot to mention the PFNs, kind of took them as implied. So mentioning the PFNs
also will do or do you suggest a better wording?

>
>> + */
>> +#ifndef modify_prot_start_ptes
>> +static inline pte_t modify_prot_start_ptes(struct vm_area_struct *vma,
>> +		unsigned long addr, pte_t *ptep, unsigned int nr)
>> +{
>> +	pte_t pte, tmp_pte;
>> +
>> +	pte = ptep_modify_prot_start(vma, addr, ptep);
>> +	while (--nr) {
>> +		ptep++;
>> +		addr += PAGE_SIZE;
>> +		tmp_pte = ptep_modify_prot_start(vma, addr, ptep);
>> +		if (pte_dirty(tmp_pte))
>> +			pte = pte_mkdirty(pte);
>> +		if (pte_young(tmp_pte))
>> +			pte = pte_mkyoung(pte);
>> +	}
>> +	return pte;
>> +}
>> +#endif
>> +
>> +/**
>> + * modify_prot_commit_ptes - Commit an update to a batch of ptes, leaving any
>> + * hardware-controlled bits in the PTE unmodified.
>> + *
>> + * @vma: The virtual memory area the pages are mapped into.
>> + * @addr: Address the first page is mapped at.
>> + * @ptep: Page table pointer for the first entry.
>> + * @old_pte: Old page table entry (for the first entry) which is now cleared.
>> + * @pte: New page table entry to be set.
>> + * @nr: Number of entries.
>> + *
>> + * May be overridden by the architecture; otherwise, implemented as a simple
>> + * loop over ptep_modify_prot_commit().
>> + *
>> + * Context: The caller holds the page table lock. The PTEs are all in the same
>> + * PMD. On exit, the set ptes in the batch map the same folio. The pte may have
>> + * been "upgraded" w.r.t a/d bits compared to the old_pte, as in, it may have
>> + * a/d bits on which were off in old_pte.
> Same comment as for ptep_modify_prot_start().
>
>> + */
>> +#ifndef modify_prot_commit_ptes
>> +static inline 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)
>> +{
>> +	int i;
>> +
>> +	for (i = 0; i < nr; ++i) {
>> +		ptep_modify_prot_commit(vma, addr, ptep, old_pte, pte);
>> +		ptep++;
>> +		addr += PAGE_SIZE;
>> +		old_pte = pte_next_pfn(old_pte);
>> +		pte = pte_next_pfn(pte);
>> +	}
>> +}
>> +#endif
>> +
>>   #endif /* CONFIG_MMU */
>>   
>>   /*
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index af10a7fbe6b8..627b0d67cc4a 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -206,7 +206,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   					continue;
>>   			}
>>   
>> -			oldpte = ptep_modify_prot_start(vma, addr, pte);
>> +			oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes);
> You're calling this with nr_ptes = 0 for the prot_numa case. But the
> implementation expects minimum nr_ptes == 1.

This will get fixed when I force nr_ptes = 1 in the previous patch right?

>
>>   			ptent = pte_modify(oldpte, newprot);
>>   
>>   			if (uffd_wp)
>> @@ -232,7 +232,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>   			    can_change_pte_writable(vma, addr, ptent))
>>   				ptent = pte_mkwrite(ptent, vma);
>>   
>> -			ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
>> +			modify_prot_commit_ptes(vma, addr, pte, oldpte, ptent, nr_ptes);
>>   			if (pte_needs_flush(oldpte, ptent))
>>   				tlb_flush_pte_range(tlb, addr, PAGE_SIZE);
>>   			pages++;