Re: [PATCH v3 1/4] mm: fix missing page-table destructor in pagetable_free()

[Anshuman Khandual]

On Fri, Oct 23, 2026 at 02:34:25PM +0800, Muchun Song wrote:
> Since commit 49f599666420 ("mm: call ctor/dtor for kernel PTEs"),
> pte_alloc_one_kernel() runs the page-table constructor for kernel PTE
> tables.
> 
> HVO (HugeTLB Vmemmap Optimization) uses pte_alloc_one_kernel() when
> splitting a PMD. Restoring the vmemmap backing pages does not collapse
> the PTE table, so a later memory hot-remove eventually frees it through
> pagetable_free() on x86-64. That releases the page without running the
> matching destructor and leaves NR_PAGETABLE elevated.
> 
> Make pagetable_free() the common free-side primitive for page-table
> pages. It checks PageTable() and runs the destructor only when the
> constructor state is still present. Callers that explicitly ran
> pagetable_dtor() have already cleared PageTable(), so they are not
> destructed twice; pages that never ran a constructor are freed directly.
> 
> Fixes: 49f599666420 ("mm: call ctor/dtor for kernel PTEs")
> Cc: stable@xxxxxxxxxxxxxxx # bf9e4e30f353: x86/mm: use pagetable_free()
> Cc: stable@xxxxxxxxxxxxxxx # 39406c05f8f1: x86/mm: Fix freeing of PMD-sized vmemmap pages
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Reviewed-by: Jose A. Perez de Azpillaga <azpijr@xxxxxxxxx>
> Reviewed-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
> Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>

> ---
> v3:
> - Explain why pagetable_free() does not run the destructor twice
>   (requested by Kevin Brodsky)
> - Add x86 stable prerequisites (suggested by
>   Jose A. Perez de Azpillaga)
> - Collect Acked-by from Dave Hansen and David Hildenbrand
> - Collect Reviewed-by from Jose A. Perez de Azpillaga and Kevin
>   Brodsky
> 
> v2:
> - Move destructor handling from x86 into pagetable_free() (suggested by
>   David Hildenbrand)
> - Trim and update the commit message (requested by Dave Hansen)
> ---
>  include/linux/mm.h | 37 ++++++++++++++++++++-----------------
>  1 file changed, 20 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dd09c438fa23..466875c5908b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3812,23 +3812,6 @@ static inline void pagetable_free_kernel(struct ptdesc *pt)
>  	__pagetable_free(pt);
>  }
>  #endif
> -/**
> - * pagetable_free - Free pagetables
> - * @pt:	The page table descriptor
> - *
> - * pagetable_free frees the memory of all page tables described by a page
> - * table descriptor and the memory for the descriptor itself.
> - */
> -static inline void pagetable_free(struct ptdesc *pt)
> -{
> -	if (ptdesc_test_kernel(pt)) {
> -		ptdesc_clear_kernel(pt);
> -		pagetable_free_kernel(pt);
> -	} else {
> -		__pagetable_free(pt);
> -	}
> -}
> -
>  #if defined(CONFIG_SPLIT_PTE_PTLOCKS)
>  #if ALLOC_SPLIT_PTLOCKS
>  void __init ptlock_cache_init(void);
> @@ -3921,6 +3904,26 @@ static inline void pagetable_dtor(struct ptdesc *ptdesc)
>  	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
>  }
>  
> +/**
> + * pagetable_free - Free pagetables
> + * @pt:	The page table descriptor
> + *
> + * pagetable_free frees the memory of all page tables described by a page
> + * table descriptor and the memory for the descriptor itself.
> + */
> +static inline void pagetable_free(struct ptdesc *pt)
> +{
> +	if (PageTable(ptdesc_page(pt)))
> +		pagetable_dtor(pt);
> +
> +	if (ptdesc_test_kernel(pt)) {
> +		ptdesc_clear_kernel(pt);
> +		pagetable_free_kernel(pt);
> +	} else {
> +		__pagetable_free(pt);
> +	}
> +}
> +
>  static inline void pagetable_dtor_free(struct ptdesc *ptdesc)
>  {
>  	pagetable_dtor(ptdesc);
> -- 
> 2.54.0
>