Re: [PATCH 3/3] ptdesc: Account page tables to memcgs again

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Feb 25, 2026 at 04:22:17PM +0000, Matthew Wilcox (Oracle) wrote:
> Commit f0c92726e89f removed the accounting of page tables to memcgs.
> Reintroduce it.
> 
> Fixes: f0c92726e89f (ptdesc: remove references to folios from __pagetable_ctor() and pagetable_dtor())
> Reported-by: Axel Rasmussen <axelrasmussen@google.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/mm.h       | 15 +++++++++++++--
>  include/linux/mm_types.h |  6 +++---
>  2 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5be3d8a8f806..34bc6f00ed7b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3519,21 +3519,32 @@ static inline unsigned long ptdesc_nr_pages(const struct ptdesc *ptdesc)
>  	return compound_nr(ptdesc_page(ptdesc));
>  }
>  
> +static inline struct mem_cgroup *pagetable_memcg(const struct ptdesc *ptdesc)
> +{
> +#ifdef CONFIG_MEMCG
> +	return ptdesc->pt_memcg;
> +#else
> +	return NULL;
> +#endif
> +}
> +
>  static inline void __pagetable_ctor(struct ptdesc *ptdesc)
>  {
>  	pg_data_t *pgdat = NODE_DATA(memdesc_nid(ptdesc->pt_flags));
> +	struct mem_cgroup *memcg = pagetable_memcg(ptdesc);
>  
>  	__SetPageTable(ptdesc_page(ptdesc));
> -	mod_node_page_state(pgdat, NR_PAGETABLE, ptdesc_nr_pages(ptdesc));
> +	memcg_stat_mod(memcg, 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));
> +	struct mem_cgroup *memcg = pagetable_memcg(ptdesc);
>  
>  	ptlock_free(ptdesc);
>  	__ClearPageTable(ptdesc_page(ptdesc));
> -	mod_node_page_state(pgdat, NR_PAGETABLE, -ptdesc_nr_pages(ptdesc));
> +	memcg_stat_mod(memcg, pgdat, NR_PAGETABLE, -ptdesc_nr_pages(ptdesc));
>  }
>  
>  static inline void pagetable_dtor_free(struct ptdesc *ptdesc)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 3cc8ae722886..e9b1da04938a 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -564,7 +564,7 @@ FOLIO_MATCH(compound_head, _head_3);
>   * @ptl:              Lock for the page table.
>   * @__page_type:      Same as page->page_type. Unused for page tables.
>   * @__page_refcount:  Same as page refcount.
> - * @pt_memcg_data:    Memcg data. Tracked for page tables here.
> + * @pt_memcg:         Memcg that this page table belongs to.
>   *
>   * This struct overlays struct page for now. Do not modify without a good
>   * understanding of the issues.
> @@ -602,7 +602,7 @@ struct ptdesc {
>  	unsigned int __page_type;
>  	atomic_t __page_refcount;
>  #ifdef CONFIG_MEMCG
> -	unsigned long pt_memcg_data;
> +	struct mem_cgroup *pt_memcg;

This is kernel memory, so this would be struct obj_cgroup * instead of struct
mem_cgroup pointer. We will need something similar to __folio_objcg(), maybe
__ptdesc_objcg() and then call obj_cgroup_memcg() on it. Basically how
folio_memcg() handles the kernel memory.