Re: [PATCH v3 14/18] mm/huge_memory: clean up after-split folio freeing in __folio_split

[David Hildenbrand (Arm)]

On 8/20/26 20:55, Kairui Song via B4 Relay wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
> 
> Replace free_folio_and_swap_cache() with an explicit folio_free_swap()
> and folio_put() in the after-split loop.  free_folio_and_swap_cache()
> unlocks the folio, then free_swap_cache() must trylock it again and
> re-check folio_mapped() before freeing the swap cache entries; if the
> trylock loses a race, the entries are left behind even though the folio
> reference is dropped.  The sub folios are still locked and unmapped
> here, so just directly call folio_free_swap() directly under the lock,
> unlock and drop the reference.  This makes the swap cache freeing
> deterministic and the reference drop explicit.
> 
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  mm/huge_memory.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 84c6e4bbaa88..113a33cddace 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4337,14 +4337,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  		if (new_folio == page_folio(lock_at))
>  			continue;
>  
> -		folio_unlock(new_folio);
>  		/*
>  		 * Subpages whose mapping has been zapped may be freed
>  		 * earlier, but freeing them requires taking the
> -		 * lru_lock, so we defer put_page() on tail pages until
> +		 * lru_lock, so we defer folio_put() on tail pages until
>  		 * after the split completes.
>  		 */
> -		free_folio_and_swap_cache(new_folio);
> +		if (is_swapcache && !folio_mapped(new_folio))
> +			folio_free_swap(new_folio);

But this is effectively duplicating code that belongs into swap code.

Should we instead have a helper next to free_folio_and_swap_cache that consumes
an already locked folio?

-- 
Cheers,

David