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

[Zi Yan <ziy@nvidia.com>]

On 3 Nov 2025, at 19:36, Wei Yang wrote:

> On Mon, Nov 03, 2025 at 11:34:47AM -0500, Zi Yan wrote:
>> On 31 Oct 2025, at 22:11, 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
>>
>> Please elaborate on what the check is for.
>>
>>> proceeding with further validation.
>
> How about this:
>
> The only functional difference is that uniform_split_supported() includes an
> additional check on the requested @new_order and split type to confirm support
> from file system or swap cache.

You are describing what the code does instead of its actual meaning.
You need to describe:
1. what is the difference between uniform split and non-uniform split?
2. what order does what file systems support? Only order-0.
3. what order does swap cache support? Only order-0.
4. why can uniform split be used to split large folios from 2 or 3 to
   order-0?
5. why can non uniform split not be used to split large folios from 2
   or 3 to order-0?
6. The logic similarity between uniform_split_supported() and
   non_uniform_split_supported() and they can be combined with detailed
   comment.

>
>>>
>>> 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.
>>> +			 */
>>
>> Blindly copying the comment here causes fusion. The checks for
>> uniform and non uniform look similar but this comment is specific
>> for non uniform split. The “No split” only applies to non uniform
>> split, but for uniform split as long as order is 0, the folio
>> can be split.
>>
>
> Per my understanding, "no split" applies to both uniform/non uniform split
> when new_order is not 0.

Not exactly. For non uniform split, any new_order value is not allowed.

>
> So the logic here is:
>
>   * uniform split && !new_order: no more check
>   * non uniform split: do the check regardless of the new_order
>
> But I am lack of some background knowledge, if it is wrong, please correct me.

You are changing the code, please do your homework first. Or you can
ask. After go through the above 6 bullet points, you should get the
background knowledge.

>
>> Please rewrite this comment to clarify both uniform and non uniform
>> cases.
>
> Not sure this one would be better?
>
>    We can always split a folio down to a single page (new_order == 0) directly.

Not always, the exceptions are listed below.

>
>    For any other scenario
>       * uniform split targeting a large folio (new_order > 0)
>       * any non-uniform split
>    we must confirm that the file system supports large folios.
>
>    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.

These filesystems do not support large folios except THPs created from khugepaged when CONFIG_READ_ONLY_THP_FOR_FS is enabled.

>
>>>  			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 */
>>
>> The same issue like the above one. Please rewrite this comment as well.
>>
>
> How about this one:
>
>    swapcache folio could only be split to order 0

This looks good.

>
>    For non-uniform split or uniform split targeting a large folio, return
>    false.

You are just describing the code.

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 (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);
>>> -- 
>>> 2.34.1
>>
>>
>> Best Regards,
>> Yan, Zi
>
> -- 
> Wei Yang
> Help you, Help me


Best Regards,
Yan, Zi