Re: [PATCH 2/3] mm/memory: batch set uffd-wp markers during zapping

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

On 16/07/26 3:11 pm, David Hildenbrand (Arm) wrote:
> On 7/15/26 13:18, Dev Jain wrote:
>> Enable batch setting of uffd-wp ptes.
>>
>> The code paths passing nr > 1 to zap_install_uffd_wp_if_needed() produce
>> that nr through either folio_pte_batch or swap_pte_batch, therefore
>> batching is correct:
>>
>> 1) all ptes belong to the same type of VMA (anonymous or non-anonymous,
>>    wp-armed or non-wp-armed)
>>
>> 2) all ptes being marked with uffd-wp or all being not marked (same is the
>>    case with the pte_swp_uffd_wp_any check)
>>
>> 3) uffd_supports_wp_marker() is independent of the function parameters
>>
>> Note that we will have to use set_pte_at() in a loop instead of set_ptes()
>> since the latter cannot handle present->non-present conversion for
>> nr_pages > 1.
>>
>> Rename the function to cond_install_uffd_wp_ptes.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>> To handle nonpresent->nonpresent transition in the ptes, we can have a
>> set_nonpresent_ptes() (in my unmap series) : if !softleaf_has_pfn(), use
>> set the same pte value to all ptep's in the patch. if softleaf_has_pfn(),
>> then add a softleaf_next_pfn() to construct the next softleaf, and
>> pte_next_softleaf() to call softleaf_next_pfn() and preserve the
>> wp bit, s-d bit, etc from the previous pte.
>>
>>  include/linux/mm.h |  6 +++--
>>  mm/memory.c        | 64 +++++++++++++++++-----------------------------
>>  mm/rmap.c          |  2 +-
>>  3 files changed, 29 insertions(+), 43 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index a71341c44655e..94e0a92bc70b1 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -5406,6 +5406,8 @@ void map_anon_folio_pte_nopf(struct folio *folio, pte_t *pte,
>>  		struct vm_area_struct *vma, unsigned long addr,
>>  		bool uffd_wp);
>>  
>> -bool pte_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>> -		unsigned long addr, pte_t *ptep, pte_t pte);
>> +bool cond_install_uffd_wp_ptes(struct vm_area_struct *vma, unsigned long addr,
>> +			       pte_t *ptep, pte_t pte,
>> +			       unsigned long nr_ptes);
> 
> 
> Two tab ...

Ok.


> 
>> +
>>  #endif /* _LINUX_MM_H */
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 98b3ace15cef2..5d2b567b383d4 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1676,27 +1676,29 @@ static inline bool zap_drop_markers(struct zap_details *details)
>>  }
>>  
>>  /**
>> - * pte_install_uffd_wp_if_needed - install uffd-wp marker after clearing a PTE
>> - * @vma: The VMA the page is mapped into.
>> - * @addr: Address the page is mapped at.
>> - * @ptep: Page table pointer for this entry.
>> + * cond_install_uffd_wp_ptes - install uffd-wp markers after clearing PTEs
>> + * @vma: The VMA the pages are mapped into.
>> + * @addr: Address the first page of this batch is mapped at.
>> + * @ptep: Page table pointer for the first entry of this batch.
>>   * @pte: Old value of the entry pointed to by @ptep.
>> + * @nr_ptes: Number of entries to install.
>>   *
>> - * If the PTE was write-protected by uffd-wp in any form, arm a special PTE
>> - * to replace a none PTE. NOTE! This should only be called when the PTE is
>> - * already cleared so we will never accidentally replace something valuable.
>> - * Meanwhile none PTEs also mean we are not demoting the PTE so a TLB flush is
>> - * not needed. E.g., when the PTE was cleared, the caller should have taken care
>> - * of the TLB flush.
>> + * If the PTEs were write-protected by uffd-wp in any form, arm special
>> + * PTEs to replace none PTEs. NOTE! This should only be called when the PTEs
>> + * are already cleared so we will never accidentally replace something
>> + * valuable. Meanwhile none PTEs also mean we are not demoting the PTEs so a
>> + * TLB flush is not needed. E.g., when PTEs were cleared, the caller should
>> + * have taken care of the TLB flush.
>>   *
>> - * Must be called with the page table lock held so that no thread will see the
>> - * none PTE, and if they see it, they'll fault and serialize at the page table
>> - * lock.
>> + * Must be called with the page table lock held so that no thread will see
>> + * the none PTEs, and if they see them, they'll fault and serialize at the
>> + * page table lock.
>>   *
>> - * Returns true if an uffd-wp PTE was installed, false otherwise.
>> + * Returns true if uffd-wp PTEs were installed, false otherwise.
>>   */
>> -bool pte_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>> -				   unsigned long addr, pte_t *ptep, pte_t pte)
>> +bool cond_install_uffd_wp_ptes(struct vm_area_struct *vma,
>> +		unsigned long addr, pte_t *ptep, pte_t pte,
>> +		unsigned long nr_ptes)
>>  {
>>  	bool arm_uffd_pte = false;
>>  
>> @@ -1726,13 +1728,14 @@ bool pte_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>>  	if (unlikely(pte_swp_uffd_any(pte)))
>>  		arm_uffd_pte = true;
>>  
>> -	if (unlikely(arm_uffd_pte)) {
>> +	if (likely(!arm_uffd_pte))
>> +		return false;
>> +
>> +	for (unsigned long i = 0; i < nr_ptes; ++i, ++ptep, addr += PAGE_SIZE)
>>  		set_pte_at(vma->vm_mm, addr, ptep,
>>  			   make_pte_marker(PTE_MARKER_UFFD_WP));
>> -		return true;
>> -	}
> 
> Can we keep the  "for (;;)" style of iterating PTEs that we use elsewhere?

Ok.


> 
>>  
>> -	return false;
>> +	return true;
>>  }
>>  
>>  /*
>> @@ -1746,29 +1749,10 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>>  			      unsigned long addr, pte_t *pte, int nr,
>>  			      struct zap_details *details, pte_t pteval)
>>  {
>> -	bool was_installed = false;
>> -
>> -	if (!uffd_supports_wp_marker())
>> -		return false;
>> -
>> -	/* Zap on anonymous always means dropping everything */
>> -	if (vma_is_anonymous(vma))
>> -		return false;
>> -
>>  	if (zap_drop_markers(details))
>>  		return false;
>>  
>> -	for (;;) {
>> -		/* the PFN in the PTE is irrelevant. */
>> -		if (pte_install_uffd_wp_if_needed(vma, addr, pte, pteval))
>> -			was_installed = true;
>> -		if (--nr == 0)
>> -			break;
>> -		pte++;
>> -		addr += PAGE_SIZE;
>> -	}
>> -
>> -	return was_installed;
>> +	return cond_install_uffd_wp_ptes(vma, addr, pte, pteval, nr);
>>  }
>>  
>>  static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index ad820fe86f7d8..2f938d0ac6953 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -2345,7 +2345,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>>  		 * we may want to replace a none pte with a marker pte if
>>  		 * it's file-backed, so we don't lose the tracking info.
>>  		 */
>> -		pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
>> +		cond_install_uffd_wp_ptes(vma, address, pvmw.pte, pteval, 1);
> 
> Was about to ask whether we should provide a wrapper, but the next patch
> converts this case.
>