Re: [PATCH v4 1/3] mm: add get_and_clear_ptes() and clear_ptes()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Jul 24, 2025 at 10:52:59AM +0530, Dev Jain wrote:
> From: David Hildenbrand <david@redhat.com>
>
> Let's add variants to be used where "full" does not apply -- which will
> be the majority of cases in the future. "full" really only applies if
> we are about to tear down a full MM.
>
> Use get_and_clear_ptes() in existing code, clear_ptes() users will
> be added next.
>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  arch/arm64/mm/mmu.c     |  2 +-
>  include/linux/pgtable.h | 45 +++++++++++++++++++++++++++++++++++++++++
>  mm/mremap.c             |  2 +-
>  mm/rmap.c               |  2 +-
>  4 files changed, 48 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index abd9725796e9..20a89ab97dc5 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1528,7 +1528,7 @@ early_initcall(prevent_bootmem_remove_init);
>  pte_t modify_prot_start_ptes(struct vm_area_struct *vma, unsigned long addr,
>  			     pte_t *ptep, unsigned int nr)
>  {
> -	pte_t pte = get_and_clear_full_ptes(vma->vm_mm, addr, ptep, nr, /* full = */ 0);
> +	pte_t pte = get_and_clear_ptes(vma->vm_mm, addr, ptep, nr);
>
>  	if (alternative_has_cap_unlikely(ARM64_WORKAROUND_2645198)) {
>  		/*
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index e3b99920be05..4c035637eeb7 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -736,6 +736,29 @@ static inline pte_t get_and_clear_full_ptes(struct mm_struct *mm,
>  }
>  #endif
>
> +/**
> + * get_and_clear_ptes - Clear present PTEs that map consecutive pages of
> + *			the same folio, collecting dirty/accessed bits.
> + * @mm: Address space 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 to clear.
> + *
> + * Use this instead of get_and_clear_full_ptes() if it is known that we don't
> + * need to clear the full mm, which is mostly the case.
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * 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.
> + */
> +static inline pte_t get_and_clear_ptes(struct mm_struct *mm, unsigned long addr,
> +		pte_t *ptep, unsigned int nr)
> +{
> +	return get_and_clear_full_ptes(mm, addr, ptep, nr, 0);
> +}
> +
>  #ifndef clear_full_ptes
>  /**
>   * clear_full_ptes - Clear present PTEs that map consecutive pages of the same
> @@ -768,6 +791,28 @@ static inline void clear_full_ptes(struct mm_struct *mm, unsigned long addr,
>  }
>  #endif
>
> +/**
> + * clear_ptes - Clear present PTEs that map consecutive pages of the same folio.
> + * @mm: Address space 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 to clear.
> + *
> + * Use this instead of clear_full_ptes() if it is known that we don't need to
> + * clear the full mm, which is mostly the case.
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might be write-protected.
> + *
> + * 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.
> + */
> +static inline void clear_ptes(struct mm_struct *mm, unsigned long addr,
> +		pte_t *ptep, unsigned int nr)
> +{
> +	clear_full_ptes(mm, addr, ptep, nr, 0);
> +}
> +
>  /*
>   * If two threads concurrently fault at the same page, the thread that
>   * won the race updates the PTE and its local TLB/Cache. The other thread
> diff --git a/mm/mremap.c b/mm/mremap.c
> index ac39845e9718..677a4d744df9 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -280,7 +280,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
>  							 old_pte, max_nr_ptes);
>  			force_flush = true;
>  		}
> -		pte = get_and_clear_full_ptes(mm, old_addr, old_ptep, nr_ptes, 0);
> +		pte = get_and_clear_ptes(mm, old_addr, old_ptep, nr_ptes);
>  		pte = move_pte(pte, old_addr, new_addr);
>  		pte = move_soft_dirty_pte(pte);
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index f93ce27132ab..568198e9efc2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2036,7 +2036,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			flush_cache_range(vma, address, end_addr);
>
>  			/* Nuke the page table entry. */
> -			pteval = get_and_clear_full_ptes(mm, address, pvmw.pte, nr_pages, 0);
> +			pteval = get_and_clear_ptes(mm, address, pvmw.pte, nr_pages);
>  			/*
>  			 * We clear the PTE but do not flush so potentially
>  			 * a remote CPU could still be writing to the folio.
> --
> 2.30.2
>