Re: [PATCH v4 05/16] mm/hugetlb: use direct assignment instead of folio_change_private()

[Zi Yan]

On 13 Sep 2026, at 22:24, Zi Yan wrote:

> folio_change_private() should be used along with folio_attach_private() and
> folio_detach_private(), where adding and remove ->private content requires
> folio refcount change. add_hugetlb_folio() simply sets folio->private to
> NULL without refcount manipulation. Change it to direct assignment to avoid
> semantic confusion.
>
> It prepares for a future commit that remove PG_private.
>
> No functional change intended.
>
> Assisted-by: LLM
> To: Muchun Song <muchun.song@xxxxxxxxx>
> To: Oscar Salvador <osalvador@xxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
> Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d3a0650ff6905..8d551cdaef9db 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1446,11 +1446,8 @@ void add_hugetlb_folio(struct hstate *h, struct folio *folio,
>  	}
>
>  	__folio_set_hugetlb(folio);
> -	folio_change_private(folio, NULL);
> -	/*
> -	 * We have to set hugetlb_vmemmap_optimized again as above
> -	 * folio_change_private(folio, NULL) cleared it.
> -	 */
> +	/* Clear all folio->private flags except hugetlb_vmemmap_optimized. */
> +	folio->private = NULL;
>  	folio_set_hugetlb_vmemmap_optimized(folio);
>

Sashiko[1] said when a CMA hugetlb folio is freed but fails vmemmap restore,
add_hugetlb_folio() clears HPG_cma. It either bypasses
hugetlb_cma_free_frozen_folio() due to the missing HPG_cma flag, or
skips cma_release() and causes a CMA memory leak if it is free directly
to buddy allocator. Sashiko also asked if HPG_temporary should be preserved
as well HPG_cma.

Answer:

Missing HPG_cma is a real issue. It can be fixed by checking
folio_test_hugetlb_cma(folio) at the beginning of add_hugetlb_folio() and
setting HPG_cma after folio->private is assigned to NULL.


HPG_temporary does not need to be preserved, since the folio is added
back to the surplus pool.

I will send a patch to fix the missing HPG_cma with
Fixes: d2d7867140806 ("mm/hugetlb: enable bootmem allocation from CMA areas")


[1] https://sashiko.dev/#/patchset/73521?part=5

Best Regards,
Yan, Zi