Re: [PATCH v3 1/4] mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split to >0 order

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Oct 21, 2025 at 11:35:27PM -0400, Zi Yan wrote:
> folio split clears PG_has_hwpoisoned, but the flag should be preserved in
> after-split folios containing pages with PG_hwpoisoned flag if the folio is
> split to >0 order folios. Scan all pages in a to-be-split folio to
> determine which after-split folios need the flag.
>
> An alternatives is to change PG_has_hwpoisoned to PG_maybe_hwpoisoned to
> avoid the scan and set it on all after-split folios, but resulting false
> positive has undesirable negative impact. To remove false positive, caller
> of folio_test_has_hwpoisoned() and folio_contain_hwpoisoned_page() needs to
> do the scan. That might be causing a hassle for current and future callers
> and more costly than doing the scan in the split code. More details are
> discussed in [1].
>
> It is OK that current implementation does not do this, because memory
> failure code always tries to split to order-0 folios and if a folio cannot
> be split to order-0, memory failure code either gives warnings or the split
> is not performed.
>
> Link: https://lore.kernel.org/all/CAHbLzkoOZm0PXxE9qwtF4gKR=cpRXrSrJ9V9Pm2DJexs985q4g@mail.gmail.com/ [1]
> Signed-off-by: Zi Yan <ziy@nvidia.com>

I guess this was split out to [0]? :)

[0]: https://lore.kernel.org/linux-mm/44310717-347c-4ede-ad31-c6d375a449b9@linux.dev/

> ---
>  mm/huge_memory.c | 28 +++++++++++++++++++++++++---
>  1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index fc65ec3393d2..f3896c1f130f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3455,6 +3455,17 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>  					caller_pins;
>  }
>
> +static bool page_range_has_hwpoisoned(struct page *first_page, long nr_pages)
> +{
> +	long i;
> +
> +	for (i = 0; i < nr_pages; i++)
> +		if (PageHWPoison(first_page + i))
> +			return true;
> +
> +	return false;
> +}
> +
>  /*
>   * It splits @folio into @new_order folios and copies the @folio metadata to
>   * all the resulting folios.
> @@ -3462,22 +3473,32 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>  static void __split_folio_to_order(struct folio *folio, int old_order,
>  		int new_order)
>  {
> +	/* Scan poisoned pages when split a poisoned folio to large folios */
> +	bool check_poisoned_pages = folio_test_has_hwpoisoned(folio) &&
> +				    new_order != 0;
>  	long new_nr_pages = 1 << new_order;
>  	long nr_pages = 1 << old_order;
>  	long i;
>
> +	folio_clear_has_hwpoisoned(folio);
> +
> +	/* Check first new_nr_pages since the loop below skips them */
> +	if (check_poisoned_pages &&
> +	    page_range_has_hwpoisoned(folio_page(folio, 0), new_nr_pages))
> +		folio_set_has_hwpoisoned(folio);
>  	/*
>  	 * Skip the first new_nr_pages, since the new folio from them have all
>  	 * the flags from the original folio.
>  	 */
>  	for (i = new_nr_pages; i < nr_pages; i += new_nr_pages) {
>  		struct page *new_head = &folio->page + i;
> -
>  		/*
>  		 * Careful: new_folio is not a "real" folio before we cleared PageTail.
>  		 * Don't pass it around before clear_compound_head().
>  		 */
>  		struct folio *new_folio = (struct folio *)new_head;
> +		bool poisoned_new_folio = check_poisoned_pages &&
> +			page_range_has_hwpoisoned(new_head, new_nr_pages);
>
>  		VM_BUG_ON_PAGE(atomic_read(&new_folio->_mapcount) != -1, new_head);
>
> @@ -3514,6 +3535,9 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  				 (1L << PG_dirty) |
>  				 LRU_GEN_MASK | LRU_REFS_MASK));
>
> +		if (poisoned_new_folio)
> +			folio_set_has_hwpoisoned(new_folio);
> +
>  		new_folio->mapping = folio->mapping;
>  		new_folio->index = folio->index + i;
>
> @@ -3600,8 +3624,6 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  	int start_order = uniform_split ? new_order : old_order - 1;
>  	int split_order;
>
> -	folio_clear_has_hwpoisoned(folio);
> -
>  	/*
>  	 * split to new_order one order at a time. For uniform split,
>  	 * folio is split to new_order directly.
> --
> 2.51.0
>