Re: [PATCH] mm: avoid poison consumption when splitting THP

[Zi Yan <ziy@nvidia.com>]

On 10 Sep 2025, at 22:14, Andrew Zaborowski wrote:

> Handling a memory failure pointing inside a huge page requires splitting
> the page.  The splitting logic uses a mechanism, implemented in
> migrate.c:try_to_map_unused_to_zeropage(), that inspects contents of
> individual pages to find zero-filled pages.  The read access to the
> contents may cause a new, synchronous exception like an x86 Machine
> Check, delivered before the initial memory_failure() finishes, ending
> in a crash.
>
> Luckily memory_failure() already sets the has_hwpoisoned flag on the
> folio right before try_to_split_thp_page().  Don't enable the shared
> zeropage mechanism (RMP_USE_SHARED_ZEROPAGE flag) down in
> __split_unmapped_folio() when the original folio has has_hwpoisoned.
>
> Note: we're disabling a potentially useful feature, some of the
> individual pages that aren't poisoned might be zero-filled.  One
> argument for not trying to add a mechanism to maybe re-scan them later,
> apart from code cost, is that the owning process is likely being
> killed and the memory released.

Sounds reasonable to me.

>
> Signed-off-by: Andrew Zaborowski <balrogg+code@gmail.com>
> ---
>  mm/huge_memory.c    | 3 ++-
>  mm/memory-failure.c | 6 ++++--
>  2 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9c38a95e9f0..1568f0308b9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3588,6 +3588,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  		struct list_head *list, bool uniform_split)
>  {
>  	struct deferred_split *ds_queue = get_deferred_split_queue(folio);
> +	bool has_hwpoisoned = folio_test_has_hwpoisoned(folio);

The state needs to be stored here because __split_unmapped_folio()
clears the flag. Maybe add a comment here to prevent people
from “optimizing” it by calling folio_test_has_hwpoisoned(folio)
in the code below.

(I wanted to until I checked the definition of folio_test_has_hwpoisoned())

>  	XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>  	struct folio *end_folio = folio_next(folio);
>  	bool is_anon = folio_test_anon(folio);
> @@ -3858,7 +3859,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>  	if (nr_shmem_dropped)
>  		shmem_uncharge(mapping->host, nr_shmem_dropped);
>
> -	if (!ret && is_anon)
> +	if (!ret && is_anon && !has_hwpoisoned)
>  		remap_flags = RMP_USE_SHARED_ZEROPAGE;
>  	remap_page(folio, 1 << order, remap_flags);
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index fc30ca4804b..2d755493de9 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -2352,8 +2352,10 @@ int memory_failure(unsigned long pfn, int flags)
>  		 * otherwise it may race with THP split.
>  		 * And the flag can't be set in get_hwpoison_page() since
>  		 * it is called by soft offline too and it is just called
> -		 * for !MF_COUNT_INCREASED.  So here seems to be the best
> -		 * place.
> +		 * for !MF_COUNT_INCREASED.
> +		 * It also tells __split_unmapped_folio() to not bother

s/__split_unmapped_folio/__folio_split/, since remap_page() is
called in __folio_split().

> +		 * using the shared zeropage -- the all-zeros check would
> +		 * consume the poison.  So here seems to be the best place.
>  		 *
>  		 * Don't need care about the above error handling paths for
>  		 * get_hwpoison_page() since they handle either free page
> -- 
> 2.45.2

Otherwise, Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi