Re: [PATCH v2 2/4] mm/huge_memory: replace can_split_folio() with direct refcount calculation

[David Hildenbrand (Red Hat)]

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@xxxxxxxxxx>
> > Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> > ---
> >   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