Re: [PATCH v4] mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split to >0 order

[Lorenzo Stoakes]

On Wed, Oct 22, 2025 at 11:05:21PM -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].
>
> This issue can be exposed via:
> 1. splitting a has_hwpoisoned folio to >0 order from debugfs interface;
> 2. truncating part of a has_hwpoisoned folio in
>    truncate_inode_partial_folio().
>
> And later accesses to a hwpoisoned page could be possible due to the
> missing has_hwpoisoned folio flag. This will lead to MCE errors.
>
> Link: https://lore.kernel.org/all/CAHbLzkoOZm0PXxE9qwtF4gKR=cpRXrSrJ9V9Pm2DJexs985q4g@xxxxxxxxxxxxxx/ [1]
> Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>

This seems reasonable to me and is a good spot (thanks!), so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
> From V3[1]:
>
> 1. Separated from the original series;
> 2. Added Fixes tag and cc'd stable;
> 3. Simplified page_range_has_hwpoisoned();
> 4. Renamed check_poisoned_pages to handle_hwpoison, made it const, and
>    shorten the statement;
> 5. Removed poisoned_new_folio variable and checked the condition
>    directly.
>
> [1] https://lore.kernel.org/all/20251022033531.389351-2-ziy@xxxxxxxxxx/
>
>  mm/huge_memory.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index fc65ec3393d2..5215bb6aecfc 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3455,6 +3455,14 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
>  					caller_pins;
>  }
>
> +static bool page_range_has_hwpoisoned(struct page *page, long nr_pages)
> +{
> +	for (; nr_pages; page++, nr_pages--)
> +		if (PageHWPoison(page))
> +			return true;
> +	return false;
> +}
> +
>  /*
>   * It splits @folio into @new_order folios and copies the @folio metadata to
>   * all the resulting folios.
> @@ -3462,17 +3470,24 @@ 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 */
> +	const bool handle_hwpoison = folio_test_has_hwpoisoned(folio) && new_order;

OK was going to mention has_hwpoisoned is FOLIO_SECOND_PAGE but looks like you
already deal with that :)

>  	long new_nr_pages = 1 << new_order;
>  	long nr_pages = 1 << old_order;
>  	long i;
>
> +	folio_clear_has_hwpoisoned(folio);

OK so we start by clearing the HW poisoned flag for the folio as a whole, which
amounts to &folio->page[1] (which must be a tail page of course as new_order
tested above).

No other pages in the range should have this flag set as is a folio thing only.

But this, in practice, sets the has_hwpoisoned flag for the first split folio...

> +
> +	/* Check first new_nr_pages since the loop below skips them */
> +	if (handle_hwpoison &&
> +	    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;
> -

NIT: Why are we removing this newline?

>  		/*
>  		 * Careful: new_folio is not a "real" folio before we cleared PageTail.
>  		 * Don't pass it around before clear_compound_head().
> @@ -3514,6 +3529,10 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  				 (1L << PG_dirty) |
>  				 LRU_GEN_MASK | LRU_REFS_MASK));
>
> +		if (handle_hwpoison &&
> +		    page_range_has_hwpoisoned(new_head, new_nr_pages))
> +			folio_set_has_hwpoisoned(new_folio);
> +

...We then, for each folio which will be split, we check again and propagate to
each based on pages in range.

>  		new_folio->mapping = folio->mapping;
>  		new_folio->index = folio->index + i;
>
> @@ -3600,8 +3619,6 @@ 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
>