Re: [PATCH v2 1/4] mm: fix missing page-table destructor in pagetable_free()

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

On 10/10/26 13:24, 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() detect constructor-backed pages and run the
> destructor before releasing their memory to fix the problem.
> 
> Fixes: 49f599666420 ("mm: call ctor/dtor for kernel PTEs")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
> 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 5cca90fc1df6..771a7d60b0a5 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3969,23 +3969,6 @@ 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);
> @@ -4078,6 +4061,26 @@ 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);


We do have

static inline void pagetable_dtor_free(struct ptdesc *ptdesc)
{
	pagetable_dtor(ptdesc);
	pagetable_free(ptdesc);
}

... can we now get rid of pagetable_dtor_free() and simply use pagetable_free() ?

(separate cleanup)

> +
> +	if (ptdesc_test_kernel(pt)) {
> +		ptdesc_clear_kernel(pt);
> +		pagetable_free_kernel(pt);
> +	} else {
> +		__pagetable_free(pt);
> +	}


This goes into the right direction. I'm still confused, but the less freeing
functions we have the better.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David