Re: [PATCH 2/4] mm: Account pagetable memory when allocated

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

On Thu, Nov 13, 2025 at 02:04:44PM +0000, Matthew Wilcox (Oracle) wrote:
> Move the accounting from the constructor to the allocation site.
> Some of the architecture code is a little complex to reason about,

I think the patch is correct. After taking a look at all the paths, I
noticed 2 cases that I'm uncertain about:

1) Commit 454b0289c6b5 ("sparc32: mm: Don't try to free page-table pages
if ctor() fails") implies that sparc32 uses memblock pages, so we never
actually free them with pagetable_free(). I'm not sure how the
allocation path looks. Would this break the accounting? +Cc-ing Will.

2) In !CONFIG_MMU_GATHER_TABLE_FREE, tlb_remove_table() calls
pagetable_dtor() -> tlb_remove_page(). I'm wondering if this may run into
accounting issues?
Commit f21bb37afbba0 ("mm: pgtable: make generic tlb_remove_table() use
struct ptdesc") says it's only an arm thing that needs to be fixed
anyway. +Cc-ing Qi.

> but I think this is all correct (and slightly more efficient due
> to having 'order' as an argument instead of having to retrieve it
> from struct page again).
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/mm.h | 11 -----------
>  mm/memory.c        | 16 +++++++++++++---
>  2 files changed, 13 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e168ee23091e..17f783c04c87 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3082,26 +3082,15 @@ 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 unsigned long ptdesc_nr_pages(const struct ptdesc *ptdesc)
> -{
> -	return compound_nr(ptdesc_page(ptdesc));
> -}
> -
>  static inline void __pagetable_ctor(struct ptdesc *ptdesc)
>  {
> -	pg_data_t *pgdat = NODE_DATA(memdesc_nid(ptdesc->pt_flags));
> -
>  	__SetPageTable(ptdesc_page(ptdesc));
> -	mod_node_page_state(pgdat, NR_PAGETABLE, ptdesc_nr_pages(ptdesc));
>  }
>  
>  static inline void pagetable_dtor(struct ptdesc *ptdesc)
>  {
> -	pg_data_t *pgdat = NODE_DATA(memdesc_nid(ptdesc->pt_flags));
> -
>  	ptlock_free(ptdesc);
>  	__ClearPageTable(ptdesc_page(ptdesc));
> -	mod_node_page_state(pgdat, NR_PAGETABLE, -ptdesc_nr_pages(ptdesc));
>  }
>  
>  static inline void pagetable_dtor_free(struct ptdesc *ptdesc)
> diff --git a/mm/memory.c b/mm/memory.c
> index 781cd7f607f7..35886fde189c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -7351,7 +7351,13 @@ long copy_folio_from_user(struct folio *dst_folio,
>  struct ptdesc *pagetable_alloc_noprof(gfp_t gfp, unsigned int order)
>  {
>  	struct page *page = alloc_frozen_pages_noprof(gfp | __GFP_COMP, order);
> +	pg_data_t *pgdat;
>  
> +	if (!page)
> +		return NULL;
> +
> +	pgdat = NODE_DATA(page_to_nid(page));
> +	mod_node_page_state(pgdat, NR_PAGETABLE, 1 << order);
>  	return page_ptdesc(page);
>  }
>  
> @@ -7364,12 +7370,16 @@ struct ptdesc *pagetable_alloc_noprof(gfp_t gfp, unsigned int order)
>   */
>  void pagetable_free(struct ptdesc *pt)
>  {
> +	pg_data_t *pgdat = NODE_DATA(memdesc_nid(pt->pt_flags));
>  	struct page *page = ptdesc_page(pt);
> +	unsigned int order = compound_order(page);
>  
> -	if (ptdesc_test_kernel(pt))
> +	if (ptdesc_test_kernel(pt)) {
>  		pagetable_free_kernel(pt);
> -	else
> -		free_frozen_pages(page, compound_order(page));
> +		return;
> +	}
> +	mod_node_page_state(pgdat, NR_PAGETABLE, -(1L << order));
> +	free_frozen_pages(page, order);
>  }
>  
>  #if defined(CONFIG_SPLIT_PTE_PTLOCKS) && ALLOC_SPLIT_PTLOCKS
> -- 
> 2.47.2
>