Re: [PATCH v3 1/4] mm/huge_memory: change folio_split_supported() to folio_check_splittable()

[Balbir Singh <balbirs@nvidia.com>]

On 11/26/25 14:50, Zi Yan wrote:
> folio_split_supported() used in try_folio_split_to_order() requires
> folio->mapping to be non NULL, but current try_folio_split_to_order() does
> not check it. There is no issue in the current code, since
> try_folio_split_to_order() is only used in truncate_inode_partial_folio(),
> where folio->mapping is not NULL.
> 
> To prevent future misuse, move folio->mapping NULL check (i.e., folio is
> truncated) into folio_split_supported(). Since folio->mapping NULL check
> returns -EBUSY and folio_split_supported() == false means -EINVAL, change
> folio_split_supported() return type from bool to int and return error
> numbers accordingly. Rename folio_split_supported() to
> folio_check_splittable() to match the return type change.
> 
> While at it, move is_huge_zero_folio() check and folio_test_writeback()
> check into folio_check_splittable() and add kernel-doc.
> 
> Remove all warnings inside folio_check_splittable() and give warnings
> in __folio_split() instead, so that bool warns parameter can be removed.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
> ---
>  include/linux/huge_mm.h |  6 ++--
>  mm/huge_memory.c        | 76 +++++++++++++++++++++++------------------
>  2 files changed, 46 insertions(+), 36 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 1d439de1ca2c..66105a90b4c3 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -375,8 +375,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
>  int folio_split_unmapped(struct folio *folio, unsigned int new_order);
>  int min_order_for_split(struct folio *folio);
>  int split_folio_to_list(struct folio *folio, struct list_head *list);
> -bool folio_split_supported(struct folio *folio, unsigned int new_order,
> -		enum split_type split_type, bool warns);
> +int folio_check_splittable(struct folio *folio, unsigned int new_order,
> +			   enum split_type split_type);
>  int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>  		struct list_head *list);
>  
> @@ -407,7 +407,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
>  static inline int try_folio_split_to_order(struct folio *folio,
>  		struct page *page, unsigned int new_order)
>  {
> -	if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
> +	if (folio_check_splittable(folio, new_order, SPLIT_TYPE_NON_UNIFORM))
>  		return split_huge_page_to_order(&folio->page, new_order);
>  	return folio_split(folio, new_order, page, NULL);
>  }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 041b554c7115..771df0c02a4a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3688,15 +3688,40 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  	return 0;
>  }
>  
> -bool folio_split_supported(struct folio *folio, unsigned int new_order,
> -		enum split_type split_type, bool warns)
> +/**
> + * folio_check_splittable() - check if a folio can be split to a given order
> + * @folio: folio to be split
> + * @new_order: the smallest order of the after split folios (since buddy
> + *             allocator like split generates folios with orders from @folio's
> + *             order - 1 to new_order).
> + * @split_type: uniform or non-uniform split
> + *
> + * folio_check_splittable() checks if @folio can be split to @new_order using
> + * @split_type method. The truncated folio check must come first.
> + *
> + * Context: folio must be locked.
> + *
> + * Return: 0 - @folio can be split to @new_order, otherwise an error number is
> + * returned.
> + */
> +int folio_check_splittable(struct folio *folio, unsigned int new_order,
> +			   enum split_type split_type)
>  {
> +	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> +	/*
> +	 * Folios that just got truncated cannot get split. Signal to the
> +	 * caller that there was a race.
> +	 *
> +	 * TODO: this will also currently refuse folios without a mapping in the
> +	 * swapcache (shmem or to-be-anon folios).
> +	 */
> +	if (!folio_test_anon(folio) && !folio->mapping)
> +		return -EBUSY;
> +

Nit: Shouldn't the order of check be 

if (!folio->mapping && !folio_test_anon(folio))

works better if folio->mapping is NULL


>  	if (folio_test_anon(folio)) {
>  		/* order-1 is not supported for anonymous THP. */
> -		VM_WARN_ONCE(warns && new_order == 1,
> -				"Cannot split to order-1 folio");
>  		if (new_order == 1)
> -			return false;
> +			return -EINVAL;
>  	} else if (split_type == SPLIT_TYPE_NON_UNIFORM || new_order) {
>  		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>  		    !mapping_large_folio_support(folio->mapping)) {
> @@ -3717,9 +3742,7 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
>  			 * case, the mapping does not actually support large
>  			 * folios properly.
>  			 */
> -			VM_WARN_ONCE(warns,
> -				"Cannot split file folio to non-0 order");
> -			return false;
> +			return -EINVAL;
>  		}
>  	}
>  
> @@ -3732,12 +3755,16 @@ bool folio_split_supported(struct folio *folio, unsigned int new_order,
>  	 * here.
>  	 */
>  	if ((split_type == SPLIT_TYPE_NON_UNIFORM || new_order) && folio_test_swapcache(folio)) {
> -		VM_WARN_ONCE(warns,
> -			"Cannot split swapcache folio to non-0 order");
> -		return false;
> +		return -EINVAL;
>  	}
>  
> -	return true;
> +	if (is_huge_zero_folio(folio))
> +		return -EINVAL;
> +
> +	if (folio_test_writeback(folio))
> +		return -EBUSY;
> +
> +	return 0;
>  }
>  
>  static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int new_order,
> @@ -3922,7 +3949,6 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	int remap_flags = 0;
>  	int extra_pins, ret;
>  	pgoff_t end = 0;
> -	bool is_hzp;
>  
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
>  	VM_WARN_ON_ONCE_FOLIO(!folio_test_large(folio), folio);
> @@ -3930,31 +3956,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	if (folio != page_folio(split_at) || folio != page_folio(lock_at))
>  		return -EINVAL;
>  
> -	/*
> -	 * Folios that just got truncated cannot get split. Signal to the
> -	 * caller that there was a race.
> -	 *
> -	 * TODO: this will also currently refuse shmem folios that are in the
> -	 * swapcache.
> -	 */
> -	if (!is_anon && !folio->mapping)
> -		return -EBUSY;
> -
>  	if (new_order >= old_order)
>  		return -EINVAL;
>  
> -	if (!folio_split_supported(folio, new_order, split_type, /* warn = */ true))
> -		return -EINVAL;
> -
> -	is_hzp = is_huge_zero_folio(folio);
> -	if (is_hzp) {
> -		pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
> -		return -EBUSY;
> +	ret = folio_check_splittable(folio, new_order, split_type);
> +	if (ret) {
> +		VM_WARN_ONCE(ret == -EINVAL, "Tried to split an unsplittable folio");
> +		return ret;
>  	}
>  
> -	if (folio_test_writeback(folio))
> -		return -EBUSY;
> -
>  	if (is_anon) {
>  		/*
>  		 * The caller does not necessarily hold an mmap_lock that would

Otherwise,looks good!

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