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

[Zi Yan <ziy@nvidia.com>]

On 22 Oct 2025, at 16:09, David Hildenbrand wrote:

> On 22.10.25 05:35, 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.
>>
>
> We're losing PG_has_hwpoisoned for large folios, so likely this should be
> a stable fix for splitting anything to an order > 0 ?

I was the borderline on this, because:

1. before the hotfix, which prevents silently bumping target split order,
   memory failure would give a warning when a folio is split to >0 order
   folios. The warning is masking this issue.
2. after the hotfix, folios with PG_has_hwpoisoned will not be split
   to >0 order folios since memory failure always wants to split a folio
   to order 0 and a folio containing LBS folios will not be split, thus
   without losing PG_has_hwpoisoned.

But one can use debugfs interface to split a has_hwpoisoned folio to >0 order
folios.

I will add
Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
and cc stable in the next version.

>
>> Link: https://lore.kernel.org/all/CAHbLzkoOZm0PXxE9qwtF4gKR=cpRXrSrJ9V9Pm2DJexs985q4g@mail.gmail.com/ [1]
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   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;
>
> Nit: I'd just do
>
> static bool page_range_has_hwpoisoned(struct page *page, unsigned long nr_pages)
> {
> 	for (; nr_pages; page++, nr_pages--)
> 		if (PageHWPoison(page))
> 			return true;
> 	}
> 	return false;
> }
>

OK, will use this one.

>> +}
>> +
>>   /*
>>    * 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;
>
> I'd shorten this to "handle_hwpoison" or sth like that.
>
> Maybe we can make it const and fit it into a single line.
>
> Comparison with 0 is not required.
>
> 	const bool handle_hwpoison = folio_test_has_hwpoisoned(folio) && new_order;

Sure, will use this.

>
>>   	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);
>
> Is the temp variable really required? I'm afraid it is a bit ugly either way :)
>
> I'd just move it into the if() below.
>
> 	if (handle_hwpoison &&
> 	    page_range_has_hwpoisoned(new_head, new_nr_pages)
> 		folio_set_has_hwpoisoned(new_folio);
>

Sure. :)

--
Best Regards,
Yan, Zi