Re: [RFC PATCH v5 01/15] mm: Consolidate freeing of typed folios on final folio_put()

[David Hildenbrand <david@redhat.com>]

On 17.01.25 17:29, Fuad Tabba wrote:
> Some folio types, such as hugetlb, handle freeing their own
> folios. Moreover, guest_memfd will require being notified once a
> folio's reference count reaches 0 to facilitate shared to private
> folio conversion, without the folio actually being freed at that
> point.
> 
> As a first step towards that, this patch consolidates freeing
> folios that have a type. The first user is hugetlb folios. Later
> in this patch series, guest_memfd will become the second user of
> this.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>   include/linux/page-flags.h | 15 +++++++++++++++
>   mm/swap.c                  | 24 +++++++++++++++++++-----
>   2 files changed, 34 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 691506bdf2c5..6615f2f59144 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -962,6 +962,21 @@ static inline bool page_has_type(const struct page *page)
>   	return page_mapcount_is_type(data_race(page->page_type));
>   }
>   
> +static inline int page_get_type(const struct page *page)
> +{
> +	return page->page_type >> 24;
> +}
> +
> +static inline bool folio_has_type(const struct folio *folio)
> +{
> +	return page_has_type(&folio->page);
> +}
> +
> +static inline int folio_get_type(const struct folio *folio)
> +{
> +	return page_get_type(&folio->page);
> +}
> +
>   #define FOLIO_TYPE_OPS(lname, fname)					\
>   static __always_inline bool folio_test_##fname(const struct folio *folio) \
>   {									\
> diff --git a/mm/swap.c b/mm/swap.c
> index 10decd9dffa1..6f01b56bce13 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -94,6 +94,20 @@ static void page_cache_release(struct folio *folio)
>   		unlock_page_lruvec_irqrestore(lruvec, flags);
>   }
>   
> +static void free_typed_folio(struct folio *folio)
> +{
> +	switch (folio_get_type(folio)) {
> +	case PGTY_hugetlb:
> +		free_huge_folio(folio);
> +		return;
> +	case PGTY_offline:
> +		/* Nothing to do, it's offline. */
> +		return;

Please drop the PGTY_offline part for now, it was rather to highlight 
what could be done.

But the real goal will be to not make offline pages
use the refcount at all (frozen).

If we really want the temporary PGTY_offline change, it should be 
introduced separately.

Apart from that LGTM!

-- 
Cheers,

David / dhildenb