Re: [Patch v3 4/4] mm/huge_memory: optimize old_order derivation during folio splitting

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

On Fri, Oct 24, 2025 at 11:29:00AM -0400, Zi Yan wrote:
>On 24 Oct 2025, at 10:46, Lorenzo Stoakes wrote:
[...]
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 093b3ffb180f..a4fa8b0e5b5a 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3596,8 +3596,8 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>  		struct address_space *mapping, bool uniform_split)
>>>  {
>>>  	const bool is_anon = folio_test_anon(folio);
>>> -	int order = folio_order(folio);
>>> -	int start_order = uniform_split ? new_order : order - 1;
>>> +	int old_order = folio_order(folio);
>>> +	int start_order = uniform_split ? new_order : old_order - 1;
>>>  	int split_order;
>>>
>>>  	folio_clear_has_hwpoisoned(folio);
>>> @@ -3609,14 +3609,11 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>  	for (split_order = start_order;
>>>  	     split_order >= new_order;
>>
>> A thought for the future - now things are simplified, it might be nice to just
>> separate out the core of this loop and have the uniform split just call the
>> split out function directly, and the non-uniform one do the loop.
>>
>> As it's a bit gross in the uniform case we just let split_order go to new_order
>> - 1 to exit the loop.
>
>Yeah, something like:
>
>if (uniform_split) {
>	if (mapping)
>		xas_split(xas, folio, old_order);
>	split_folio_to_order(...);
>	return 0;
>}
>
>for () {
>...
>split_folio_to_order(...);
>...
>}
>
>where split_folio_to_order(...) just
>split memcg, split page owner, pgalloc_tag_split, __split_folio_to_order,
>and stats update
>

This looks reasonable, while I found we already have split_folio_to_order().

>>
>> BUT - let's please save that for another patch :)
>
>I agree.
>>
>> This all looks fine.
>>
>>>  	     split_order--) {
>>> -		int old_order = folio_order(folio);
>>>  		int nr_new_folios = 1UL << (old_order - split_order);
>>>
>>>  		/* order-1 anonymous folio is not supported */
>>>  		if (is_anon && split_order == 1)
>>>  			continue;
>>> -		if (uniform_split && split_order != new_order)
>>> -			continue;
>>>
>>>  		if (mapping) {
>>>  			/*
>>> @@ -3643,7 +3640,13 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>  			mod_mthp_stat(old_order, MTHP_STAT_NR_ANON, -1);
>>>  			mod_mthp_stat(split_order, MTHP_STAT_NR_ANON, nr_new_folios);
>>>  		}
>>> +		/*
>>> +		 * If uniform split, the process is complete.
>>> +		 * If non-uniform, continue splitting the folio at @split_at
>>> +		 * as long as the next @split_order is >= @new_order.
>>> +		 */
>>>  		folio = page_folio(split_at);
>>> +		old_order = split_order;
>>>  	}
>>>
>>>  	return 0;
>>> --
>>> 2.34.1
>>>
>>>
>
>
>--
>Best Regards,
>Yan, Zi

-- 
Wei Yang
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