Re: [PATCH] mm/huge_memory: Fix outdated comment about freeing subpages in __folio_split

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/23/26 05:49, Barry Song (Xiaomi) wrote:
> The comment appears to be outdated. First, add_to_swap() no longer
> exists after Kairui’s commit b487a2da3575 ("mm, swap: simplify folio
> swap allocation"). Second, partially zapped folios are now always
> split before folio_alloc_swap() to avoid extra I/O, following Ryan’s
> commit 5ed890ce5147 ("mm: vmscan: avoid split during
> shrink_folio_list()").

Not necessarily. We have some scenarios where we have partially-mapped folios
not detected as such as of today (with CONFIG_NO_PAGE_MAPCOUNT).

> Fix this by making the description more generic.
> 
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: Kairui Song <kasong@tencent.com>
> Cc: Kemeng Shi <shikemeng@huaweicloud.com>
> Cc: Nhat Pham <nphamcs@gmail.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Youngjun Park <youngjun.park@lge.com>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/huge_memory.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 970e077019b7..4586f3ccb133 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4190,11 +4190,10 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  
>  		folio_unlock(new_folio);
>  		/*
> -		 * Subpages may be freed if there wasn't any mapping
> -		 * like if add_to_swap() is running on a lru page that
> -		 * had its mapping zapped. And freeing these pages
> -		 * requires taking the lru_lock so we do the put_page
> -		 * of the tail pages after the split is complete.
> +		 * 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
> +		 * after the split completes.
>  		 */
>  		free_folio_and_swap_cache(new_folio);
>  	}

Acked-by: David Hildenbrand (Arm) <david@kernel.org>


-- 
Cheers,

David