Re: [PATCH v7 7/8] mm: Introduce deferred freeing for kernel page tables

[Mike Rapoport <rppt@kernel.org>]

On Wed, Oct 22, 2025 at 04:26:33PM +0800, Lu Baolu wrote:
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> This introduces a conditional asynchronous mechanism, enabled by
> CONFIG_ASYNC_KERNEL_PGTABLE_FREE. When enabled, this mechanism defers the
> freeing of pages that are used as page tables for kernel address mappings.
> These pages are now queued to a work struct instead of being freed
> immediately.
> 
> This deferred freeing allows for batch-freeing of page tables, providing
> a safe context for performing a single expensive operation (TLB flush)
> for a batch of kernel page tables instead of performing that expensive
> operation for each page table.
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  mm/Kconfig           |  3 +++
>  include/linux/mm.h   | 16 +++++++++++++---
>  mm/pgtable-generic.c | 37 +++++++++++++++++++++++++++++++++++++
>  3 files changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 0e26f4fc8717..a83df9934acd 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -908,6 +908,9 @@ config PAGE_MAPCOUNT
>  config PGTABLE_HAS_HUGE_LEAVES
>  	def_bool TRANSPARENT_HUGEPAGE || HUGETLB_PAGE
>  
> +config ASYNC_KERNEL_PGTABLE_FREE
> +	def_bool n
> +
>  # TODO: Allow to be enabled without THP
>  config ARCH_SUPPORTS_HUGE_PFNMAP
>  	def_bool n
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 52ae551d0eb4..d521abd33164 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3031,6 +3031,14 @@ static inline void __pagetable_free(struct ptdesc *pt)
>  	__free_pages(page, compound_order(page));
>  }
>  
> +#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE
> +void pagetable_free_kernel(struct ptdesc *pt);
> +#else
> +static inline void pagetable_free_kernel(struct ptdesc *pt)
> +{
> +	__pagetable_free(pt);
> +}
> +#endif
>  /**
>   * pagetable_free - Free pagetables
>   * @pt:	The page table descriptor
> @@ -3040,10 +3048,12 @@ static inline void __pagetable_free(struct ptdesc *pt)
>   */
>  static inline void pagetable_free(struct ptdesc *pt)
>  {
> -	if (ptdesc_test_kernel(pt))
> +	if (ptdesc_test_kernel(pt)) {
>  		ptdesc_clear_kernel(pt);
> -
> -	__pagetable_free(pt);
> +		pagetable_free_kernel(pt);
> +	} else {
> +		__pagetable_free(pt);
> +	}
>  }
>  
>  #if defined(CONFIG_SPLIT_PTE_PTLOCKS)
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index 567e2d084071..1c7caa8ef164 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -406,3 +406,40 @@ pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
>  	pte_unmap_unlock(pte, ptl);
>  	goto again;
>  }
> +
> +#ifdef CONFIG_ASYNC_KERNEL_PGTABLE_FREE
> +static void kernel_pgtable_work_func(struct work_struct *work);
> +
> +static struct {
> +	struct list_head list;
> +	/* protect above ptdesc lists */
> +	spinlock_t lock;
> +	struct work_struct work;
> +} kernel_pgtable_work = {
> +	.list = LIST_HEAD_INIT(kernel_pgtable_work.list),
> +	.lock = __SPIN_LOCK_UNLOCKED(kernel_pgtable_work.lock),
> +	.work = __WORK_INITIALIZER(kernel_pgtable_work.work, kernel_pgtable_work_func),
> +};
> +
> +static void kernel_pgtable_work_func(struct work_struct *work)
> +{
> +	struct ptdesc *pt, *next;
> +	LIST_HEAD(page_list);
> +
> +	spin_lock(&kernel_pgtable_work.lock);
> +	list_splice_tail_init(&kernel_pgtable_work.list, &page_list);
> +	spin_unlock(&kernel_pgtable_work.lock);
> +
> +	list_for_each_entry_safe(pt, next, &page_list, pt_list)
> +		__pagetable_free(pt);
> +}
> +
> +void pagetable_free_kernel(struct ptdesc *pt)
> +{
> +	spin_lock(&kernel_pgtable_work.lock);
> +	list_add(&pt->pt_list, &kernel_pgtable_work.list);
> +	spin_unlock(&kernel_pgtable_work.lock);
> +
> +	schedule_work(&kernel_pgtable_work.work);
> +}
> +#endif
> -- 
> 2.43.0
> 

-- 
Sincerely yours,
Mike.