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

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

On Sat, Oct 25, 2025 at 11:30:19AM -0400, Zi Yan wrote:
> On 24 Oct 2025, at 11:44, Lorenzo Stoakes wrote:
>
> > 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@mail.gmail.com/ [1]
> >> Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
> >> Cc: stable@vger.kernel.org
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >
> > This seems reasonable to me and is a good spot (thanks!), so:
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >
> >> ---
> >> 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@nvidia.com/
> >>
> >>  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 :)
>
> Right. And has_hwpoisoned is only set for large folios.

Yup this is what I meant by you already dealing with it :)

>
> >
> >>  	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...
>
> handle_hwpoison is only true when after-split folios are large (new_order not 0).
> All folio has_hwpoisoned set code is guarded by handle_hwpoison.

Yup I know, maybe I should have been explciit :)

>
> >
> >> +
> >> +	/* 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?
>
> It is a newline between two declarations.

Oh you're right, sorry!

>
> >
> >>  		/*
> >>  		 * 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.
>
> Yes, but this loop only goes [new_nr_pages, nr_pages), so the code above is
> needed for [0, new_nr_pages). The loop is done in this way to avoid redundant
> work, flag and compound head setting, for [0, new_nr_pages) pages and the
> original folio, since there is no change between the original values and
> after-split values.

Yup I know, was just working through the logic, it looks right to me!

>
> >
> >>  		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
> >>
>
>
> --
> Best Regards,
> Yan, Zi

Thanks for doing this! :)

Cheers, Lorenzo