[PATCH 1/1] mm/hugetlb: keep max_huge_pages when dissolving surplus folios

[Longlong Xia]

From: Longlong Xia <xialonglong@xxxxxxxxxx>

dissolve_free_hugetlb_folio() can remove a free folio as surplus when
its node has surplus pages.  In that case remove_hugetlb_folio()
decrements both nr_huge_pages and surplus_huge_pages, leaving the
persistent pool size unchanged.

Updating max_huge_pages as if a persistent folio had been removed can
therefore corrupt the persistent pool target and underflow it when
max_huge_pages is zero.  Keep max_huge_pages unchanged for surplus
folios, including the vmemmap restoration rollback path.

Fixes: cb402bbdabca ("mm/hugetlb: fix surplus pages in dissolve_free_huge_page()")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>
---
 mm/hugetlb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e93c4d2456aa..2239e2566a52 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2002,7 +2002,8 @@ int dissolve_free_hugetlb_folio(struct folio *folio)
 		if (h->surplus_huge_pages_node[folio_nid(folio)])
 			adjust_surplus = true;
 		remove_hugetlb_folio(h, folio, adjust_surplus);
-		h->max_huge_pages--;
+		if (!adjust_surplus)
+			h->max_huge_pages--;
 		spin_unlock_irq(&hugetlb_lock);
 
 		/*
@@ -2022,7 +2023,8 @@ int dissolve_free_hugetlb_folio(struct folio *folio)
 			if (rc) {
 				spin_lock_irq(&hugetlb_lock);
 				add_hugetlb_folio(h, folio, adjust_surplus);
-				h->max_huge_pages++;
+				if (!adjust_surplus)
+					h->max_huge_pages++;
 				goto out;
 			}
 		} else {
-- 
2.43.0