Re: [PATCH] mm/hugetlb: copy the CMA flag when demoting

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 5/1/25 10:13, Frank van der Linden wrote:
> Since commit d2d786714080 ("mm/hugetlb: enable bootmem allocation
> from CMA areas"), a flag is used to mark hugetlb folios as allocated
> from CMA. This flag is also used to decide if it should be freed
> to CMA.
> 
> However, the flag isn't copied to the smaller folios when a hugetlb
> folio is broken up for demotion, which would cause it to be freed
> incorrectly.
> 
> Fix this by copying the flag to the smaller order hugetlb pages
> created from the original one.
> 
> Fixes: d2d786714080 ("mm/hugetlb: enable bootmem allocation from CMA areas")
> Signed-off-by: Frank van der Linden <fvdl@google.com>
> ---
>  mm/hugetlb.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e3e6ac991b9c..6ea1be71aa42 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4034,10 +4034,13 @@ static long demote_free_hugetlb_folios(struct hstate *src, struct hstate *dst,
>  
>  	list_for_each_entry_safe(folio, next, src_list, lru) {
>  		int i;
> +		bool cma;
>  
>  		if (folio_test_hugetlb_vmemmap_optimized(folio))
>  			continue;
>  
> +		cma = folio_test_hugetlb_cma(folio);
> +
>  		list_del(&folio->lru);
>  
>  		split_page_owner(&folio->page, huge_page_order(src), huge_page_order(dst));
> @@ -4053,6 +4056,9 @@ static long demote_free_hugetlb_folios(struct hstate *src, struct hstate *dst,
>  
>  			new_folio->mapping = NULL;
>  			init_new_hugetlb_folio(dst, new_folio);
> +			/* Copy the CMA flag so that it is freed correctly */
> +			if (cma)
> +				folio_set_hugetlb_cma(new_folio);
>  			list_add(&new_folio->lru, &dst_list);
>  		}
>  	}

Agree with Oscar that HPG_cma need documentation like other flags.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>