Re: [RFC PATCH 3/5] mm/hugetlb: set large_rmappable on hugetlb and avoid deferred_list handling

[Zi Yan]

On 1 Feb 2026, at 22:59, Baolin Wang wrote:

> On 1/30/26 11:48 AM, Zi Yan wrote:
>> Commit f708f6970cc9 ("mm/hugetlb: fix kernel NULL pointer dereference when
>> migrating hugetlb folio") fixed a NULL pointer dereference when
>> folio_undo_large_rmappable(), now folio_unqueue_deferred_list(), is used on
>> hugetlb to clear deferred_list. It cleared large_rmappable flag on hugetlb.
>> hugetlb is rmappable, thus clearing large_rmappable flag looks misleading.
>> Instead, reject hugetlb in folio_unqueue_deferred_list() to avoid the
>> issue.
>>
>> This prepares for code separation of compound page and folio in a follow-up
>> commit.
>>
>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>> ---
>>   mm/hugetlb.c     | 6 +++---
>>   mm/hugetlb_cma.c | 2 +-
>>   mm/internal.h    | 3 ++-
>>   3 files changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 6e855a32de3d..7466c7bf41a1 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1422,8 +1422,8 @@ static struct folio *alloc_gigantic_frozen_folio(int order, gfp_t gfp_mask,
>>   	if (hugetlb_cma_exclusive_alloc())
>>   		return NULL;
>>  -	folio = (struct folio *)alloc_contig_frozen_pages(1 << order, gfp_mask,
>> -							  nid, nodemask);
>> +	folio = page_rmappable_folio(alloc_contig_frozen_pages(1 << order, gfp_mask,
>> +							  nid, nodemask));
>>   	return folio;
>>   }
>>   #else /* !CONFIG_ARCH_HAS_GIGANTIC_PAGE || !CONFIG_CONTIG_ALLOC */
>> @@ -1859,7 +1859,7 @@ static struct folio *alloc_buddy_frozen_folio(int order, gfp_t gfp_mask,
>>   	if (alloc_try_hard)
>>   		gfp_mask |= __GFP_RETRY_MAYFAIL;
>>  -	folio = (struct folio *)__alloc_frozen_pages(gfp_mask, order, nid, nmask);
>> +	folio = page_rmappable_folio(__alloc_frozen_pages(gfp_mask, order, nid, nmask));
>>    	/*
>>   	 * If we did not specify __GFP_RETRY_MAYFAIL, but still got a
>> diff --git a/mm/hugetlb_cma.c b/mm/hugetlb_cma.c
>> index f83ae4998990..4245b5dda4dc 100644
>> --- a/mm/hugetlb_cma.c
>> +++ b/mm/hugetlb_cma.c
>> @@ -51,7 +51,7 @@ struct folio *hugetlb_cma_alloc_frozen_folio(int order, gfp_t gfp_mask,
>>   	if (!page)
>>   		return NULL;
>>  -	folio = page_folio(page);
>> +	folio = page_rmappable_folio(page);
>>   	folio_set_hugetlb_cma(folio);
>>   	return folio;
>>   }
>
> IIUC, this will break the semantics of the is_transparent_hugepage() and might trigger a split of a hugetlb folio, right?
>
> static inline bool is_transparent_hugepage(const struct folio *folio)
> {
> 	if (!folio_test_large(folio))
> 		return false;
>
> 	return is_huge_zero_folio(folio) ||
> 		folio_test_large_rmappable(folio);
> }

Oh, I missed this. I will check all folio_test_large_rmappable() callers
and filter out hugetlb if necessary.

Thank you for pointing this out.

Best Regards,
Yan, Zi