Re: [Patch v2] mm/huge_memory: merge uniform_split_supported() and non_uniform_split_supported()

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Nov 05, 2025 at 09:05:31AM +0100, David Hildenbrand (Red Hat) wrote:
>On 05.11.25 08:25, 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.
>> 
>> The reason for this check comes from the following two aspects:
>> 
>>    * some file system or swap cache just supports order-0 folio
>>    * the behavioral difference between uniform/non-uniform split
>> 
>> The behavioral difference between uniform split and non-uniform:
>> 
>>    * uniform split splits folio directly to @new_order
>>    * non-uniform split creates after-split folios with orders from
>>      folio_order(folio) - 1 to new_order.
>> 
>> This means for non-uniform split or !new_order split we should check the
>> file system and swap cache respectively.
>> 
>> This commit unifies the logic and 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>
>> Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
>> 
>> ---
>> v2:
>>    * remove need_check
>>    * update comment
>>    * add more explanation in change log
>>    * selftests/split_huge_page_test pass
>> ---
>>   include/linux/huge_mm.h |  8 ++---
>>   mm/huge_memory.c        | 70 ++++++++++++++++++-----------------------
>>   2 files changed, 33 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 381a49c5ac3f..db442e0e3a46 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3666,55 +3666,49 @@ 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;
>> -	}
>> -
>> -	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)) {
>> -		VM_WARN_ONCE(warns && new_order == 1,
>> -				"Cannot split to order-1 folio");
>> -		return new_order != 1;
>> -	} else  if (new_order) {
>> +	} else if (!uniform_split || new_order) {
>>   		if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>>   		    !mapping_large_folio_support(folio->mapping)) {
>> +			/*
>> +			 * We can always split a folio down to a single page
>> +			 * (new_order == 0) uniformly.
>> +			 *
>> +			 * For any other scenario
>> +			 *   a) uniform split targeting a large folio
>> +			 *      (new_order > 0)
>> +			 *   b) any non-uniform split
>> +			 * we must confirm that the file system supports large
>> +			 * folios.
>> +			 *
>> +			 * Note that we might still have THPs in such
>> +			 * mappings, which is created from khugepaged when
>> +			 * CONFIG_READ_ONLY_THP_FOR_FS is enabled. 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)) {
>> +	/*
>> +	 * swapcache folio could only be split to order 0
>> +	 *
>> +	 * non-uniform split creates after-split folios with orders from
>> +	 * folio_order(folio) - 1 to new_order, making it not suitable for any
>> +	 * swapcache folio split. Only uniform split to order-0 can be used
>> +	 * here.
>> +	 */
>> +	if ((!uniform_split || new_order) && folio_test_swapcache(folio)) {
>
>Staring at the existing code, how would we reach the folio_test_swapcache()
>test for anon folios?
>
>At the beginning of the function we have
>
>if (folio_test_anon()) {
>	...
>	return new_order != 1;
>}
>
>Aren't we missing a check for anon folios that are in the swapcache?
>

Hmm... haven't noticed this.

I need to do some home work on it.

>-- 
>Cheers
>
>David

-- 
Wei Yang
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