Re: [PATCH v2 2/3] mm/hugetlb: Preserve persistent flags when returning folios to the pool

[Muchun Song]

> On Nov 2, 2026, at 10:49, Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:
> 
> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> 
> add_hugetlb_folio() returns a folio to the HugeTLB pool when vmemmap
> restoration fails. It clears folio->private and restores only
> HPG_vmemmap_optimized, losing other flags that must survive the rollback.
> 
> Clearing HPG_cma causes a later free to bypass the CMA release path,
> leaving the CMA allocation bitmap set. Clearing HPG_raw_hwp_unreliable
> loses the indication that the locations of poisoned subpages are unknown,
> undermining the checks that prevent freeing or unpoisoning such folios.
> 
> Preserve the existing flags instead of resetting folio->private. Callers
> already clear HPG_restore_reserve, HPG_migratable and HPG_freed before
> returning a folio to the pool. Clear HPG_temporary in free_huge_folio()
> before attempting to free a temporary folio, so a failed vmemmap restore
> does not leave that flag set on a folio returned as surplus.
> 
> Add a VM_WARN_ON_ONCE_FOLIO() check for flags other than
> HPG_vmemmap_optimized, HPG_raw_hwp_unreliable and HPG_cma to detect
> incomplete transient flag cleanup.
> 
> Fixes: 161df60e9e89 ("mm, hwpoison, hugetlb: support saving mechanism of raw error pages")
> Fixes: d2d786714080 ("mm/hugetlb: enable bootmem allocation from CMA areas")
> Closes: https://sashiko.dev/#/bug/linux-5363eb09-9c3c-4f8b-9f3c-a998a8c4b258
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Muchun Song <muchun.song@xxxxxxxxx>
> Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

But,

> ---
> mm/hugetlb.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4a008afaedf9..931ca3a30773 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1443,6 +1443,12 @@ void add_hugetlb_folio(struct hstate *h, struct folio *folio,
> 
> 	lockdep_assert_held(&hugetlb_lock);
> 
> +	/* Callers must clear transient flags before returning to the pool. */
> +	VM_WARN_ON_ONCE_FOLIO((unsigned long)folio->private &
> +			     ~(BIT(HPG_vmemmap_optimized) |

It looks like the parameters aren't aligned. A space missing before "~"?

Thanks.

> +			       BIT(HPG_raw_hwp_unreliable) |
> +			       BIT(HPG_cma)), folio);
> +
> 	INIT_LIST_HEAD(&folio->lru);
> 	h->nr_huge_pages++;
> 	h->nr_huge_pages_node[nid]++;
> @@ -1453,9 +1459,6 @@ void add_hugetlb_folio(struct hstate *h, struct folio *folio,
> 	}
> 
> 	__folio_set_hugetlb(folio);
> -	/* Clear all folio->private flags except hugetlb_vmemmap_optimized. */
> -	folio->private = NULL;
> -	folio_set_hugetlb_vmemmap_optimized(folio);
> 
> 	arch_clear_hugetlb_flags(folio);
> 	enqueue_hugetlb_folio(h, folio);
> @@ -1743,6 +1746,7 @@ void free_huge_folio(struct folio *folio)
> 		h->resv_huge_pages++;
> 
> 	if (folio_test_hugetlb_temporary(folio)) {
> +		folio_clear_hugetlb_temporary(folio);
> 		remove_hugetlb_folio(h, folio, false);
> 		spin_unlock_irqrestore(&hugetlb_lock, flags);
> 		update_and_free_hugetlb_folio(h, folio, true);
> -- 
> 2.54.0 (Apple Git-157)
>