Re: [PATCH v2 1/5] mm: Separate folio_split_memcg_refs() from split_page_memcg()

[David Hildenbrand <david@redhat.com>]

On 14.03.25 14:36, Matthew Wilcox (Oracle) wrote:
> Folios always use memcg_data to refer to the mem_cgroup while pages
> allocated with GFP_ACCOUNT have a pointer to the obj_cgroup.  Since the
> caller already knows what it has, split the function into two and then
> we don't need to check.
> 
> Move the assignment of split folio memcg_data to the point where we set
> up the other parts of the new folio.  That leaves folio_split_memcg_refs()
> just handling the memcg accounting.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> Acked-by: Zi Yan <ziy@nvidia.com>
> ---
>   include/linux/memcontrol.h |  7 +++++++
>   mm/huge_memory.c           | 16 ++++------------
>   mm/memcontrol.c            | 17 +++++++++++++----
>   3 files changed, 24 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 57664e2a8fb7..d090089c5497 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1039,6 +1039,8 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
>   }
>   
>   void split_page_memcg(struct page *head, int old_order, int new_order);
> +void folio_split_memcg_refs(struct folio *folio, unsigned old_order,
> +		unsigned new_order);
>   
>   static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
>   {
> @@ -1463,6 +1465,11 @@ static inline void split_page_memcg(struct page *head, int old_order, int new_or
>   {
>   }
>   
> +static inline void folio_split_memcg_refs(struct folio *folio,
> +		unsigned old_order, unsigned new_order)
> +{
> +}
> +
>   static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
>   {
>   	return 0;
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 14b1963898a7..3e5ecc8f3d13 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3394,6 +3394,9 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>   			folio_set_young(new_folio);
>   		if (folio_test_idle(folio))
>   			folio_set_idle(new_folio);
> +#ifdef CONFIG_MEMCG
> +		new_folio->memcg_data = folio->memcg_data;
> +#endif
>   
>   		folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio));
>   	}
> @@ -3525,18 +3528,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>   			}
>   		}
>   
> -		/*
> -		 * Reset any memcg data overlay in the tail pages.
> -		 * folio_nr_pages() is unreliable until prep_compound_page()
> -		 * was called again.
> -		 */
> -#ifdef NR_PAGES_IN_LARGE_FOLIO
> -		folio->_nr_pages = 0;
> -#endif


I remember that we could trigger a warning without that, but I don't 
immediately find where that warning was. IIRC, if we'd split to order-0, 
page[1] would have indicated that it had a memcg set, and something 
bailed out.

Maybe Zi Yan recalls where that check fired.

In any case, if that warning no longer fires this is a very nice cleanup!

-- 
Cheers,

David / dhildenb