Re: [PATCH] mm/huge_memory: merge uniform_split_supported() and non_uniform_split_supported()

[Zi Yan <ziy@nvidia.com>]

On 3 Nov 2025, at 4:04, Dev Jain wrote:

> On 01/11/25 7:41 am, Wei Yang wrote:
>> The functions uniform_split_supported() and
>> non_uniform_split_supported() share significantly similar logic.
>>
>> The only functional difference is that uniform_split_supported()
>> includes an additional check on the requested @new_order before
>> proceeding with further validation.
>>
>> This commit unifies the logic by introducing a new variable,
>> @need_check, which is conditionally set based on whether a uniform
>> split is requested. This allows us to merge the two functions into
>> a single, combined helper, removing redundant code and simplifying
>> the split support checking mechanism.
>>
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> ---
>>   include/linux/huge_mm.h |  8 +++---
>>   mm/huge_memory.c        | 55 +++++++++++------------------------------
>>   2 files changed, 18 insertions(+), 45 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index cbb2243f8e56..79343809a7be 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -369,10 +369,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
>>   		unsigned int new_order, bool unmapped);
>>   int min_order_for_split(struct folio *folio);
>>   int split_folio_to_list(struct folio *folio, struct list_head *list);
>> -bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>> -		bool warns);
>> -bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
>> -		bool warns);
>> +bool folio_split_supported(struct folio *folio, unsigned int new_order,
>> +		bool uniform_split, bool warns);
>>   int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>>   		struct list_head *list);
>>  @@ -403,7 +401,7 @@ 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 (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
>> +	if (!folio_split_supported(folio, new_order, /* uniform_split = */ false, /* warns= */ false))
>>   		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 d1fa0d2d9b44..f6d2cb2a5ca0 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3673,55 +3673,34 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>   	return 0;
>>   }
>>  -bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
>> -		bool warns)
>> +bool folio_split_supported(struct folio *folio, unsigned int new_order,
>> +		bool uniform_split, bool warns)
>>   {
>> -	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");
>> -		return new_order != 1;
>> -	} else if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>> -	    !mapping_large_folio_support(folio->mapping)) {
>> -		/*
>> -		 * No split if the file system does not support large folio.
>> -		 * Note that we might still have THPs in such mappings due to
>> -		 * CONFIG_READ_ONLY_THP_FOR_FS. But in that case, the mapping
>> -		 * does not actually support large folios properly.
>> -		 */
>> -		VM_WARN_ONCE(warns,
>> -			"Cannot split file folio to non-0 order");
>> -		return false;
>> -	}
>> -
>> -	/* Only swapping a whole PMD-mapped folio is supported */
>> -	if (folio_test_swapcache(folio)) {
>> -		VM_WARN_ONCE(warns,
>> -			"Cannot split swapcache folio to non-0 order");
>> -		return false;
>> -	}
>> +	bool need_check = uniform_split ? new_order : true;
>>  -	return true;
>> -}
>> -
>> -/* See comments in non_uniform_split_supported() */
>> -bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>> -		bool warns)
>> -{
>>   	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");
>>   		return new_order != 1;
>> -	} else  if (new_order) {
>> +	} else if (need_check) {
>>   		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>>   		    !mapping_large_folio_support(folio->mapping)) {
>> +			/*
>> +			 * No split if the file system does not support large
>> +			 * folio.  Note that we might still have THPs in such
>> +			 * mappings due to CONFIG_READ_ONLY_THP_FOR_FS. But in
>> +			 * that case, the mapping does not actually support
>> +			 * large folios properly.
>> +			 */
>>   			VM_WARN_ONCE(warns,
>>   				"Cannot split file folio to non-0 order");
>>   			return false;
>>   		}
>>   	}
>>  -	if (new_order && folio_test_swapcache(folio)) {
>> +	/* Only swapping a whole PMD-mapped folio is supported */
>> +	if (need_check && folio_test_swapcache(folio)) {
>>   		VM_WARN_ONCE(warns,
>>   			"Cannot split swapcache folio to non-0 order");
>>   		return false;
>> @@ -3779,11 +3758,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>   	if (new_order >= old_order)
>>   		return -EINVAL;
>>  -	if (uniform_split && !uniform_split_supported(folio, new_order, true))
>> -		return -EINVAL;
>> -
>> -	if (!uniform_split &&
>> -	    !non_uniform_split_supported(folio, new_order, true))
>> +	if (!folio_split_supported(folio, new_order, uniform_split, /* warn = */ true))
>>   		return -EINVAL;
>>    	is_hzp = is_huge_zero_folio(folio);
>
> @Zi, I am confused on the code. Is there something stopping us from passing a zero new_order to
> non_uniform_split_supported()?

Yes, for filesystems without large folio support and a PMD size file-backed THP
is created via khugepaged (with CONFIG_READ_ONLY_THP_FOR_FS), the THP can only
be split to order-0. Non uniform split would create other orders and causes
issues.

Best Regards,
Yan, Zi