Re: [PATCH 2/8] mm/huge_memory: use folio_swap_entry() when splitting a swapcache folio

["Zi Yan" <ziy@nvidia.com>]

On Sun Aug 30, 2026 at 6:32 AM EDT, Tal Zussman wrote:
> __split_folio_to_order() open-codes the swap entry computation for the
> split-out folios. Use folio_swap_entry() instead.
>
> No functional change.
>
> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
> ---
>  mm/huge_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 54494c3fa983..03287f995d92 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3760,7 +3760,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  		VM_WARN_ON_ONCE_PAGE(new_folio->private, new_head);
>  
>  		if (folio_test_swapcache(folio))
> -			new_folio->swap.val = folio->swap.val + i;
> +			new_folio->swap = folio_swap_entry(folio, i);

folio_swap_entry() checks folio_nr_pages(folio) internally.
folio->_nr_pages is in first tail page of folio. If new_order is 0, it
is basically checking memcg_data in the first new_folio. And after first
iteration, folio->_nr_page will be clobbered. Without
NR_PAGES_IN_LARGE_FOLIO, folio_nr_pages() is derived from folio_order(),
which is in folio->_flags_1. It can have the same issue.

Please keep it open coded. You can add a comment like "during split,
not all folio fields are avaialbe, open code the swap calculation".

>  
>  		/* Page flags must be visible before we make the page non-compound. */
>  		smp_wmb();

-- 
Best Regards,
Yan, Zi