linux-kernel - Re: [PATCH v2 2/4] mm/huge_memory: replace can_split

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Message-ID: <5421c8d0-1151-48dc-8774-fd2e983735a5@kernel.org>
Date: Tue, 25 Nov 2025 09:55:40 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Balbir Singh <balbirs@...dia.com>, Zi Yan <ziy@...dia.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>,
 Miaohe Lin <linmiaohe@...wei.com>, Naoya Horiguchi
 <nao.horiguchi@...il.com>, Wei Yang <richard.weiyang@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] mm/huge_memory: replace can_split_folio() with
 direct refcount calculation

On 11/24/25 23:14, Balbir Singh wrote:
> On 11/22/25 13:55, Zi Yan wrote:
>> can_split_folio() is just a refcount comparison, making sure only the
>> split caller holds an extra pin. Open code it with
>> folio_expected_ref_count() != folio_ref_count() - 1. For the extra_pins
>> used by folio_ref_freeze(), add folio_cache_references() to calculate it.
>>
>> Suggested-by: David Hildenbrand (Red Hat) <david@...nel.org>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   include/linux/huge_mm.h |  1 -
>>   mm/huge_memory.c        | 43 ++++++++++++++++-------------------------
>>   mm/vmscan.c             |  3 ++-
>>   3 files changed, 19 insertions(+), 28 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index 97686fb46e30..1ecaeccf39c9 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -369,7 +369,6 @@ enum split_type {
>>   	SPLIT_TYPE_NON_UNIFORM,
>>   };
>>   
>> -bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>>   int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>   		unsigned int new_order);
>>   int folio_split_unmapped(struct folio *folio, unsigned int new_order);
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index c1f1055165dd..6c821c1c0ac3 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3455,23 +3455,6 @@ static void lru_add_split_folio(struct folio *folio, struct folio *new_folio,
>>   	}
>>   }
>>   
>> -/* Racy check whether the huge page can be split */
>> -bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>> -{
>> -	int extra_pins;
>> -
>> -	/* Additional pins from page cache */
>> -	if (folio_test_anon(folio))
>> -		extra_pins = folio_test_swapcache(folio) ?
>> -				folio_nr_pages(folio) : 0;
>> -	else
>> -		extra_pins = folio_nr_pages(folio);
>> -	if (pextra_pins)
>> -		*pextra_pins = extra_pins;
>> -	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins -
>> -					caller_pins;
>> -}
>> -
>>   static bool page_range_has_hwpoisoned(struct page *page, long nr_pages)
>>   {
>>   	for (; nr_pages; page++, nr_pages--)
>> @@ -3776,17 +3759,26 @@ int folio_check_splittable(struct folio *folio, unsigned int new_order,
>>   	return 0;
>>   }
>>   
>> +/* Number of folio references from the pagecache or the swapcache. */
>> +static unsigned int folio_cache_references(const struct folio *folio)
> 
> folio_cache_ref_count?

Yes, makes sense.

> 
>> +{
>> +	if (folio_test_anon(folio) && !folio_test_swapcache(folio))
>> +		return 0;
>> +	return folio_nr_pages(folio);
>> +}
 >> +>
> Does this belong to include/linux/mm.h with the other helpers?

Not for now I think, in particular, as we require earlier 
!folio->mapping checks to give a correct answer. Most people should be 
using folio_expected_ref_count().

-- 
Cheers

David