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

[Zi Yan <ziy@nvidia.com>]

On 23 Oct 2025, at 7:10, Pankaj Raghav wrote:

> On 10/23/25 05:05, 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;
>
> Is it easy to add a selftest in split_huge_page_test for this scenario?

Probably, but I prefer to do this in a separate memory failure test.
I think the steps are:
0. set up a SIGBUS handler,
1. get a XFS image, like split_huge_page_test does,
2. set block size > page size,
3. fault in a large folio bigger than block size,
4. madvise(MADV_HWPOISON),
5. catch SIGBUS since the folio cannot be split to order-0 and check
   the corresponding folio's has_hwpoison flag.

I will put this on my TODO list.

>
>> 2. truncating part of a has_hwpoisoned folio in
>>    truncate_inode_partial_folio().
>>
> --
> Pankaj


--
Best Regards,
Yan, Zi