Re: [PATCH v2 2/4] mm/huge_memory: replace can_split_folio() with direct refcount calculation

[Balbir Singh <balbirs@nvidia.com>]

On 11/22/25 13:55, Zi Yan wrote:
> can_split_folio() is just a refcount comparison, making sure only the
> split caller holds an extra pin. Open code it with
> folio_expected_ref_count() != folio_ref_count() - 1. For the extra_pins
> used by folio_ref_freeze(), add folio_cache_references() to calculate it.
> 
> Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  include/linux/huge_mm.h |  1 -
>  mm/huge_memory.c        | 43 ++++++++++++++++-------------------------
>  mm/vmscan.c             |  3 ++-
>  3 files changed, 19 insertions(+), 28 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 97686fb46e30..1ecaeccf39c9 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -369,7 +369,6 @@ enum split_type {
>  	SPLIT_TYPE_NON_UNIFORM,
>  };
>  
> -bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>  int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  		unsigned int new_order);
>  int folio_split_unmapped(struct folio *folio, unsigned int new_order);
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c1f1055165dd..6c821c1c0ac3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3455,23 +3455,6 @@ static void lru_add_split_folio(struct folio *folio, struct folio *new_folio,
>  	}
>  }
>  
> -/* Racy check whether the huge page can be split */
> -bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
> -{
> -	int extra_pins;
> -
> -	/* Additional pins from page cache */
> -	if (folio_test_anon(folio))
> -		extra_pins = folio_test_swapcache(folio) ?
> -				folio_nr_pages(folio) : 0;
> -	else
> -		extra_pins = folio_nr_pages(folio);
> -	if (pextra_pins)
> -		*pextra_pins = extra_pins;
> -	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins -
> -					caller_pins;
> -}
> -
>  static bool page_range_has_hwpoisoned(struct page *page, long nr_pages)
>  {
>  	for (; nr_pages; page++, nr_pages--)
> @@ -3776,17 +3759,26 @@ int folio_check_splittable(struct folio *folio, unsigned int new_order,
>  	return 0;
>  }
>  
> +/* Number of folio references from the pagecache or the swapcache. */
> +static unsigned int folio_cache_references(const struct folio *folio)

folio_cache_ref_count?

> +{
> +	if (folio_test_anon(folio) && !folio_test_swapcache(folio))
> +		return 0;
> +	return folio_nr_pages(folio);
> +}
> +

Does this belong to include/linux/mm.h with the other helpers?

>  static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
>  					     struct page *split_at, struct xa_state *xas,
>  					     struct address_space *mapping, bool do_lru,
>  					     struct list_head *list, enum split_type split_type,
> -					     pgoff_t end, int *nr_shmem_dropped, int extra_pins)
> +					     pgoff_t end, int *nr_shmem_dropped)
>  {
>  	struct folio *end_folio = folio_next(folio);
>  	struct folio *new_folio, *next;
>  	int old_order = folio_order(folio);
>  	int ret = 0;
>  	struct deferred_split *ds_queue;
> +	int extra_pins = folio_cache_references(folio);
>  
>  	VM_WARN_ON_ONCE(!mapping && end);
>  	/* Prevent deferred_split_scan() touching ->_refcount */
> @@ -3956,7 +3948,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	struct folio *new_folio, *next;
>  	int nr_shmem_dropped = 0;
>  	int remap_flags = 0;
> -	int extra_pins, ret;
> +	int ret;
>  	pgoff_t end = 0;
>  
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
> @@ -4036,7 +4028,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	 * Racy check if we can split the page, before unmap_folio() will
>  	 * split PMDs
>  	 */
> -	if (!can_split_folio(folio, 1, &extra_pins)) {
> +	if (folio_expected_ref_count(folio) != folio_ref_count(folio) - 1) {
>  		ret = -EAGAIN;
>  		goto out_unlock;
>  	}
> @@ -4059,8 +4051,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	}
>  
>  	ret = __folio_freeze_and_split_unmapped(folio, new_order, split_at, &xas, mapping,
> -						true, list, split_type, end, &nr_shmem_dropped,
> -						extra_pins);
> +						true, list, split_type, end, &nr_shmem_dropped);
>  fail:
>  	if (mapping)
>  		xas_unlock(&xas);
> @@ -4134,20 +4125,20 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   */
>  int folio_split_unmapped(struct folio *folio, unsigned int new_order)
>  {
> -	int extra_pins, ret = 0;
> +	int ret = 0;
>  
>  	VM_WARN_ON_ONCE_FOLIO(folio_mapped(folio), folio);
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_anon(folio), folio);
>  
> -	if (!can_split_folio(folio, 1, &extra_pins))
> +	if (folio_expected_ref_count(folio) != folio_ref_count(folio) - 1)
>  		return -EAGAIN;
>  
>  	local_irq_disable();
>  	ret = __folio_freeze_and_split_unmapped(folio, new_order, &folio->page, NULL,
>  						NULL, false, NULL, SPLIT_TYPE_UNIFORM,
> -						0, NULL, extra_pins);
> +						0, NULL);
>  	local_irq_enable();
>  	return ret;
>  }
> @@ -4640,7 +4631,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
>  		 * can be split or not. So skip the check here.
>  		 */
>  		if (!folio_test_private(folio) &&
> -		    !can_split_folio(folio, 0, NULL))
> +		    folio_expected_ref_count(folio) != folio_ref_count(folio))
>  			goto next;
>  
>  		if (!folio_trylock(folio))
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 92980b072121..3b85652a42b9 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1284,7 +1284,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  					goto keep_locked;
>  				if (folio_test_large(folio)) {
>  					/* cannot split folio, skip it */
> -					if (!can_split_folio(folio, 1, NULL))
> +					if (folio_expected_ref_count(folio) !=
> +					    folio_ref_count(folio) - 1)
>  						goto activate_locked;
>  					/*
>  					 * Split partially mapped folios right away.


Otherwise, LGTM
Acked-by: Balbir Singh <balbirs@nvidia.com>