Re: [PATCH] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order()

[Zi Yan <ziy@nvidia.com>]

On 6 Nov 2026, at 11:48, Lorenzo Stoakes (ARM) wrote:

> On Thu, Oct 29, 2026 at 10:08:44PM -0400, Zi Yan wrote:
>> __split_folio_to_order() calls prep_compound_page() on each new head page
>> to create a compound page after clear_compound_head(). prep_compound_page()
>> uses non atomic __SetPageHead(). It is mostly OK, since the folio is locked
>> and frozen and no one else is supposed to modify the folio's flags.
>>
>> However, code calling migration_entry_wait() can reach the folio by PFN,
>> then access its flags. It is because softleaf_entry_wait_on_locked() used
>
> Oh fun.
>
>> by migration_entry_wait() sets PG_waiters concurrently without taking a
>> reference and the frozen refcount during split cannot block it. Meanwhile,
>> __SetPageHead() does a plain read and write, so PG_waiters atomically set
>> by softleaf_entry_wait_on_locked() will be lost if the set happens after
>> __SetPageHead()'s read and before its write.
>>
>> One example code interleaving:
>>
>> CPU0: __split_folio_to_order(new_order>0)  CPU1: do_swap_page()
>>
>> 1. clear_compound_head(new_head)
>>    new_head->compound_info = 0
>>    (page_folio(new_head) == new_head now)
>>
>>                                            2. migration_entry_wait()
>>                                              softleaf_to_folio(entry)
>>                                              page_folio() -> new_head
>>                                              (not the old head any more)
>>
>> 3. prep_compound_page(new_head, new_order)
>>      __SetPageHead(new_head)
>>        tmp = new_head->flags.f  <-- load; PG_waiters == 0 in tmp
>>
>>                                            4. folio_set_waiters(new_head)
>>                                              set_bit(PG_waiters, &flags.f)
>>                                              enqueue on waitqueue, sleep
>>
>> 5.     new_head->flags.f = tmp | (1UL << PG_head)
>>        <-- store; overwrites step 4, PG_waiters == 0 again
>>
>> Later, folio_unlock(new_folio) sees no PG_waiters, skips folio_wake_bit(),
>> and CPU1 can remain blocked indefinitely.
>>
>> Fix it by adding the atomic version SetPageHead() and
>> prep_compound_page_atomic() using it. __split_folio_to_order() uses
>> prep_compound_page_atomic() to avoid the issue without adding an atomic ops
>> to other prep_compound_page() users.
>>
>> Reported-by: David Hildenbrand <david@kernel.org>
>> Closes: https://lore.kernel.org/all/DMHE8BEEER1H.2BIY52NXATPIA@nvidia.com/
>> Fixes: c010d47f107f6 ("mm: thp: split huge page to any lower order pages")
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>
> Nasty one. Some nits below but change looks sensible to me so:
>
> Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>
> BTW AI flags an hw poison thing doing the same:
>
> memory_failure() does TestSetPageHWPoison(p) on an arbitrary subpage before
> taking a reference (mm/memory-failure.c:2458), which races the same way with the
> plain flag clone and with folio_set_order()'s plain _flags_1 store.

These reference-less accessors are always making trouble. I will think
about how to deal with this one.

>
>> Cc: stable@vger.kernel.org
>> ---
>> The issue was raised by David when reviewing Ye Liu's "mm/huge_memory: use
>> READ_ONCE() when cloning folio flags during split" [1].
>>
>> [1] https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@linux.dev/
>> ---
>>  include/linux/page-flags.h |  1 +
>>  mm/huge_memory.c           | 10 +++++++++-
>>  mm/internal.h              |  2 ++
>>  mm/page_alloc.c            | 20 ++++++++++++++++++--
>>  4 files changed, 30 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index b0ddc652e76cc..08492ab3da895 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -854,6 +854,7 @@ static __always_inline int PageHead(const struct page *page)
>>  	return test_bit(PG_head, &page->flags.f);
>>  }
>>
>> +SETPAGEFLAG(Head, head, PF_ANY)
>>  __SETPAGEFLAG(Head, head, PF_ANY)
>>  __CLEARPAGEFLAG(Head, head, PF_ANY)
>>  CLEARPAGEFLAG(Head, head, PF_ANY)
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index bd44d58e86845..319b038eba89d 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3779,7 +3779,15 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>>  		 */
>>  		clear_compound_head(new_head);
>>  		if (new_order) {
>> -			prep_compound_page(new_head, new_order);
>> +			/*
>> +			 * new_head is recognized as a folio after
>> +			 * clear_compound_head().
>> +			 * softleaf_entry_wait_on_locked() can set PG_waiters
>> +			 * on it without taking a reference, so its frozen state
>> +			 * cannot prevent it. Use the atomic version to avoid
>> +			 * losing a concurrently set PG_waiters.
>> +			 */
>> +			prep_compound_page_atomic(new_head, new_order);
>>  			folio_set_large_rmappable(new_folio);
>>  		}
>>
>> diff --git a/mm/internal.h b/mm/internal.h
>> index aeebb3e49ed11..9b8864a8540aa 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -807,6 +807,8 @@ static inline void prep_compound_tail(struct page *tail,
>>  	VM_WARN_ON_ONCE(tail->private);
>>  }
>>
>> +void prep_compound_page_atomic(struct page *page, unsigned int order);
>> +
>>  #if defined CONFIG_COMPACTION || defined CONFIG_CMA
>>
>>  /*
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 143776e671abb..ace1359468046 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -682,18 +682,34 @@ static inline bool pcp_allowed_order(unsigned int order)
>>   * This usage means that zero-order pages may not be compound.
>>   */
>>
>> -void prep_compound_page(struct page *page, unsigned int order)
>> +static void __prep_compound_head_and_tail(struct page *page, unsigned int order)
>
> NIT: can this just be __prepare_compound_page()? I find the head + tail bit
> quite confusing. The __ implies it's an internal function.

Will rename it.

>
>>  {
>>  	int i;
>>  	int nr_pages = 1 << order;
>>
>> -	__SetPageHead(page);
>>  	for (i = 1; i < nr_pages; i++)
>>  		prep_compound_tail(page + i, page, order);
>>
>>  	prep_compound_head(page, order);
>>  }
>>
>
> NIT: Maybe add a comment describing this like prep_compound_page_atomic()?

Sure.

Thanks for the review.

>
>> +void prep_compound_page(struct page *page, unsigned int order)
>> +{
>> +	__SetPageHead(page);
>> +	__prep_compound_head_and_tail(page, order);
>> +}
>> +
>> +/*
>> + * PG_head is set atomically whereas the other metadata is set with plain stores.
>> + * It should be used when @page's flags can be modified concurrently,
>> + * e.g., in __split_folio_to_order().
>> + */
>> +void prep_compound_page_atomic(struct page *page, unsigned int order)
>> +{
>> +	SetPageHead(page);
>> +	__prep_compound_head_and_tail(page, order);
>> +}
>> +
>>  static inline void set_buddy_order(struct page *page, unsigned int order)
>>  {
>>  	set_page_private(page, order);
>>
>> ---
>> base-commit: d865f2677c44cb2171b833d6775cf692756fa8be
>> change-id: 20261029-fix-split_folio_to_order-race-087a2a9fbf7c
>>
>> Best regards,
>> --
>> Yan, Zi
>>
>
> --
> Cheers, Lorenzo


Best Regards,
Yan, Zi