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

["David Hildenbrand (Arm)" <david@kernel.org>]

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 ...

> +
>  #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?

>  
> -	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.

-- 
Cheers,

David