Re: [PATCH 2/3] mm/hugetlb: preserve CMA flag when returning folios to the pool

[Zi Yan <ziy@nvidia.com>]

On 28 Oct 2026, at 9:56, Muchun Song wrote:

>> On Oct 28, 2026, at 19:33, Kaitao Cheng <kaitao.cheng@linux.dev> wrote:
>>
>> From: Kaitao Cheng <chengkaitao@kylinos.cn>
>>
>> When vmemmap restoration fails, add_hugetlb_folio() returns the folio
>> to the hugetlb pool. It clears folio->private and restores only
>> HPG_vmemmap_optimized, losing HPG_cma on CMA-allocated folios.
>>
>> A later successful free then bypasses hugetlb_cma_free_frozen_folio()
>> and returns the pages through free_frozen_pages(). The CMA allocation
>> bitmap remains set, so the range is unavailable for future CMA
>> allocations.
>>
>> Save the CMA flag before resetting folio->private and restore it along
>> with the vmemmap optimization flag.
>
> We already knew this issue. Cc Zi Yan. Seems he hasn't sent a fix.

Thanks. No, I have not formed a proper patch yet.

>
> But we want a different solution. See:
>
> https://lore.kernel.org/linux-mm/A1A14368-7A43-40ED-B2AB-C2EC2C5FFB1D@linux.dev/

Feel free to grab my version from the link and add
VM_WARN_ON_ONCE_FOLIO()s Muchun suggested. You can add
Suggested-by: Zi Yan <ziy@nvidia.com> in your patch.

Thanks.

>
> Thanks,
> Muchun
>
>>
>> Fixes: d2d786714080 ("mm/hugetlb: enable bootmem allocation from CMA areas")
>> Closes: https://sashiko.dev/#/bug/linux-5363eb09-9c3c-4f8b-9f3c-a998a8c4b258
>> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
>> ---
>> mm/hugetlb.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 4a008afaedf9..8da4bab676c9 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1437,6 +1437,7 @@ void remove_hugetlb_folio(struct hstate *h, struct folio *folio,
>> void add_hugetlb_folio(struct hstate *h, struct folio *folio,
>> 		       bool adjust_surplus)
>> {
>> +	bool cma = folio_test_hugetlb_cma(folio);
>> 	int nid = folio_nid(folio);
>>
>> 	VM_BUG_ON_FOLIO(!folio_test_hugetlb_vmemmap_optimized(folio), folio);
>> @@ -1453,9 +1454,11 @@ void add_hugetlb_folio(struct hstate *h, struct folio *folio,
>> 	}
>>
>> 	__folio_set_hugetlb(folio);
>> -	/* Clear all folio->private flags except hugetlb_vmemmap_optimized. */
>> +	/* Preserve the vmemmap optimization state and CMA allocation origin. */
>> 	folio->private = NULL;
>> 	folio_set_hugetlb_vmemmap_optimized(folio);
>> +	if (cma)
>> +		folio_set_hugetlb_cma(folio);
>>
>> 	arch_clear_hugetlb_flags(folio);
>> 	enqueue_hugetlb_folio(h, folio);
>> -- 
>> 2.53.0
>>


Best Regards,
Yan, Zi