Re: [PATCH 1/3] mm: don't PTE-batch a swap-in over a hardware-poisoned subpage

[David Hildenbrand (Arm)]

On 11/11/26 14:18, Lorenzo Stoakes (ARM) wrote:
> On Wed, Nov 11, 2026 at 03:38:29AM -0800, Usama Arif wrote:
>> do_swap_page() checks PageHWPoison() on the faulting page, but the
>> large-folio batching path then maps the rest of the folio without looking
>> at the other subpages. A poisoned subpage that is not the one being
>> faulted gets mapped anyway, and the fault that would have reported it
>> never happens.
>>
>> Skip batching for such a folio and let each page take its own fault,
>> where the existing check catches it.
>>
>> Scan the subpages rather than testing the folio-level PG_has_hwpoisoned:
>> memory_failure() sets PageHWPoison() on the subpage first and only sets
>> the folio flag much later, and the bail-outs in between - for example
>> get_hwpoison_page() losing a page lifecycle race - return with the
>> subpage marked and the folio flag never set.
>>
>> Fixes: 508758960b8d ("mm: swap: entirely map large folios found in swapcache")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
>> ---
>>  mm/internal.h | 15 +++++++++++++++
>>  mm/memory.c   |  4 ++++
>>  2 files changed, 19 insertions(+)
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index cd3675c044e0a..e61c02f4019b0 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -28,6 +28,21 @@ struct folio_batch;
>>  int __copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
>>  		void *buf, int len, unsigned int gup_flags);
>>
>> +/*
>> + * Check every page of the folio for HWPoison, rather than relying on the
>> + * folio-level PG_has_hwpoisoned, which memory_failure() can leave clear.
>> + */
>> +static inline bool folio_has_any_hwpoisoned_page(const struct folio *folio)
>> +{
>> +	long nr = folio_nr_pages(folio);
>> +	long i;
>> +
>> +	for (i = 0; i < nr; i++)
>> +		if (PageHWPoison(folio_page(folio, i)))
>> +			return true;
>> +	return false;
>> +}
>> +
>>  /* mm/workingset.c */
>>  bool workingset_test_recent(void *shadow, bool file, bool *workingset,
>>  			    bool flush);
>> diff --git a/mm/memory.c b/mm/memory.c
>> index d477a4f986bc8..4b7d456332200 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5158,6 +5158,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>  		    swap_pte_batch(folio_ptep, nr, folio_pte) != nr)
>>  			goto check_folio;
>>
>> +		/* Let a poisoned subpage take its own fault. */
>> +		if (folio_has_any_hwpoisoned_page(folio))
>> +			goto check_folio;
>> +
> 
> Hm, isn't scanning every single page in the folio potentially quite costly
> here?  I thought the point of batching was to improve performance overall?

Yeah, we don't really want another helper to check for hwpoison.

If we race with concurrent hwpoison, bad luck. The slapped on memory-failure
code is full of races. That needs a proper long-term solution, not more pain to
core-mm.

So let's please try reusing existing helpers that test the large-folio bit.

-- 
Cheers,

David