[PATCH v4 07/15] mm: pgtable: introduce pagetable_dtor()

[Alexander Gordeev]

On Mon, Dec 30, 2024 at 05:07:42PM +0800, Qi Zheng wrote:
> The pagetable_p*_dtor() are exactly the same except for the handling of
> ptlock. If we make ptlock_free() handle the case where ptdesc->ptl is
> NULL and remove VM_BUG_ON_PAGE() from pmd_ptlock_free(), we can unify
> pagetable_p*_dtor() into one function. Let's introduce pagetable_dtor()
> to do this.
> 
> Later, pagetable_dtor() will be moved to tlb_remove_ptdesc(), so that
> ptlock and page table pages can be freed together (regardless of whether
> RCU is used). This prevents the use-after-free problem where the ptlock
> is freed immediately but the page table pages is freed later via RCU.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch at bytedance.com>
> Originally-by: Peter Zijlstra (Intel) <peterz at infradead.org>
...
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5d82f42ddd5cc..cad11fa10c192 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2992,6 +2992,15 @@ static inline bool ptlock_init(struct ptdesc *ptdesc) { return true; }
>  static inline void ptlock_free(struct ptdesc *ptdesc) {}
>  #endif /* defined(CONFIG_SPLIT_PTE_PTLOCKS) */
>  
> +static inline void pagetable_dtor(struct ptdesc *ptdesc)
> +{
> +	struct folio *folio = ptdesc_folio(ptdesc);
> +
> +	ptlock_free(ptdesc);
> +	__folio_clear_pgtable(folio);
> +	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
> +}
> +

If I am not mistaken, it is just pagetable_pte_dtor() rename.
What is the point in moving the code around?

>  static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
>  {
>  	struct folio *folio = ptdesc_folio(ptdesc);
> @@ -3003,15 +3012,6 @@ static inline bool pagetable_pte_ctor(struct ptdesc *ptdesc)
>  	return true;
>  }
>  
> -static inline void pagetable_pte_dtor(struct ptdesc *ptdesc)
> -{
> -	struct folio *folio = ptdesc_folio(ptdesc);
> -
> -	ptlock_free(ptdesc);
> -	__folio_clear_pgtable(folio);
> -	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
> -}
> -
>  pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp);
>  static inline pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr,
>  			pmd_t *pmdvalp)