Re: [RFC PATCH 3/5] mm/hugetlb: set large_rmappable on hugetlb and avoid deferred_list handling

[Baolin Wang]

On 1/30/26 11:48 AM, Zi Yan wrote:
> Commit f708f6970cc9 ("mm/hugetlb: fix kernel NULL pointer dereference when
> migrating hugetlb folio") fixed a NULL pointer dereference when
> folio_undo_large_rmappable(), now folio_unqueue_deferred_list(), is used on
> hugetlb to clear deferred_list. It cleared large_rmappable flag on hugetlb.
> hugetlb is rmappable, thus clearing large_rmappable flag looks misleading.
> Instead, reject hugetlb in folio_unqueue_deferred_list() to avoid the
> issue.
>
> This prepares for code separation of compound page and folio in a follow-up
> commit.
>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>   mm/hugetlb.c     | 6 +++---
>   mm/hugetlb_cma.c | 2 +-
>   mm/internal.h    | 3 ++-
>   3 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6e855a32de3d..7466c7bf41a1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1422,8 +1422,8 @@ static struct folio *alloc_gigantic_frozen_folio(int order, gfp_t gfp_mask,
>   	if (hugetlb_cma_exclusive_alloc())
>   		return NULL;
>   
> -	folio = (struct folio *)alloc_contig_frozen_pages(1 << order, gfp_mask,
> -							  nid, nodemask);
> +	folio = page_rmappable_folio(alloc_contig_frozen_pages(1 << order, gfp_mask,
> +							  nid, nodemask));
>   	return folio;
>   }
>   #else /* !CONFIG_ARCH_HAS_GIGANTIC_PAGE || !CONFIG_CONTIG_ALLOC */
> @@ -1859,7 +1859,7 @@ static struct folio *alloc_buddy_frozen_folio(int order, gfp_t gfp_mask,
>   	if (alloc_try_hard)
>   		gfp_mask |= __GFP_RETRY_MAYFAIL;
>   
> -	folio = (struct folio *)__alloc_frozen_pages(gfp_mask, order, nid, nmask);
> +	folio = page_rmappable_folio(__alloc_frozen_pages(gfp_mask, order, nid, nmask));
>   
>   	/*
>   	 * If we did not specify __GFP_RETRY_MAYFAIL, but still got a
> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
> index f83ae4998990..4245b5dda4dc 100644
> --- a/mm/hugetlb_cma.c
> +++ b/mm/hugetlb_cma.c
> @@ -51,7 +51,7 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
>   	if (!page)
>   		return NULL;
>   
> -	folio = page_folio(page);
> +	folio = page_rmappable_folio(page);
>   	folio_set_hugetlb_cma(folio);
>   	return folio;
>   }

IIUC, this will break the semantics of the is_transparent_hugepage() and 
might trigger a split of a hugetlb folio, right?

static inline bool is_transparent_hugepage(const struct folio *folio)
{
	if (!folio_test_large(folio))
		return false;

	return is_huge_zero_folio(folio) ||
		folio_test_large_rmappable(folio);
}