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

[David Hildenbrand <david@redhat.com>]

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 ?

> 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;
}

> +}
> +
>   /*
>    * 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;

>   	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);




-- 
Cheers

David / dhildenb