Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

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

On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote:
> do_swap_page() checks PageHWPoison() on the faulting page, but the
> large-folio batching path then maps the rest of the folio without looking
> at the other subpages. A poisoned subpage that is not the one being
> faulted gets mapped anyway, and the fault that would have reported it
> never happens.
>
> Skip batching for such a folio and let each page take its own fault,
> where the existing check catches it.
>
> Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
> memory_failure() sets PageHWPoison() on the subpage first and only sets
> the folio flag much later, and the bail-outs in between - for example
> get_hwpoison_page() losing a page lifecycle race - return with the
> subpage marked and the folio flag never set.
>
> Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
> Cc: stable@vger.kernel.org
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  mm/internal.h | 15 +++++++++++++++
>  mm/memory.c   |  4 ++++
>  2 files changed, 19 insertions(+)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index cd3675c044e0a..e61c02f4019b0 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -28,6 +28,21 @@ struct folio_batch;
>  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
>  		void *buf, int len, unsigned int gup_flags);
>
> +/*
> + * Check every page of the folio for HWPoison, rather than relying on the
> + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
> + */
> +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
> +{
> +	long nr = folio_nr_pages(folio);
> +	long i;
> +
> +	for (i = 0; i < nr; i++)
> +		if (PageHWPoison(folio_page(folio, i)))
> +			return true;
> +	return false;
> +}
> +
>  /* mm/workingset.c */
>  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
>  			    bool flush);
> diff --git a/mm/memory.c b/mm/memory.c
> index d477a4f986bc8..4b7d456332200 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
>  			goto check_folio;
>
> +		/* Let a poisoned subpage take its own fault. */
> +		if (folio_has_any_hwpoisoned_page(folio))
> +			goto check_folio;
> +

Hm, isn't scanning every single page in the folio potentially quite costly
here?  I thought the point of batching was to improve performance overall?

It seems broken to have stuff potentially interact with hwpoison but then
to only set the folio-level flag much later?

Also why are we swapping out folios containing hwpoison pages? Shouldn't we
migrate away from the hwpoison? :) (I'm not really familiar with the hw
poison stuff)


>  		page_idx = idx;
>  		address = folio_start;
>  		ptep = folio_ptep;
> --
> 2.53.0-Meta
>

--
Cheers, Lorenzo