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

[Wei Yang <richard.weiyang@gmail.com>]

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>

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

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Wei Yang
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