Re: [PATCH v2 1/5] mm/rmap: convert page -> folio for hwpoison checks

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Fri, Jul 10, 2026 at 05:59:10AM +0000, Dev Jain wrote:
> try_to_unmap() receives hugetlb folios only from the hwpoison path.

Horrifying :)

> hugetlb_update_hwpoison() sets the hugetlb folio's head-page
> hwpoison bit, and page_vma_mapped_walk() reports the hugetlb mapping at
> the head PFN, so the previous PageHWPoison(subpage) check happened to
> work for hugetlb.
>
> For non-hugetlb folios, unmap_poisoned_folio() currently rejects large
> folios before calling try_to_unmap(). Hence it is always the case that
> if try_to_unmap_one() handles an hwpoisoned folio, then the head page is
> marked with the poison bit.
>
> Therefore, convert the poisoned subpage checks to folio_test_hwpoison().
>
> No functional change intended, except that, while at it,
> convert VM_BUG_* to VM_WARN_*.
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Dev Jain <dev.jain@arm.com>

Small nit below but generally LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/rmap.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 9939400e77c79..dbb077f8443e3 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2122,10 +2122,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  			bool anon = folio_test_anon(folio);
>
>  			/*
> -			 * The try_to_unmap() is only passed a hugetlb page
> -			 * in the case where the hugetlb page is poisoned.
> +			 * The try_to_unmap() is only passed a hugetlb folio
> +			 * in the case where the hugetlb folio contains a
> +			 * poisoned page.
>  			 */
> -			VM_BUG_ON_PAGE(!PageHWPoison(subpage), subpage);
> +			VM_WARN_ON_FOLIO(!folio_test_hwpoison(folio), folio);
>  			/*
>  			 * huge_pmd_unshare may unmap an entire PMD page.
>  			 * There is no way of knowing exactly which PMDs may
> @@ -2204,7 +2205,11 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
>  		/* Update high watermark before we lower rss */
>  		update_hiwater_rss(mm);
>
> -		if (PageHWPoison(subpage) && (flags & TTU_HWPOISON)) {
> +		/*
> +		 * With TTU_HWPOISON, we only expect small folios or hugetlb
> +		 * folios here for now.
> +		 */

I think probably we can drop this? It seems implied by the code (David comment
similarly I think? :)

> +		if (folio_test_hwpoison(folio) && (flags & TTU_HWPOISON)) {
>  			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
>  			if (folio_test_hugetlb(folio)) {
>  				hugetlb_count_sub(folio_nr_pages(folio), mm);
> --
> 2.43.0
>

Cheers, Lorenzo