[PATCH 4/4] arm64/mm: fix destructor for unconstructed hotplug page tables

[Muchun Song]

Commit c594b83457cc ("arm64: mm: call pagetable dtor when freeing
hot-removed page tables") made free_hotplug_pgtable_page()
unconditionally run the page-table destructor. This matches page tables
allocated by the arm64 mapping code, which runs the corresponding
constructors.

However, arm64 also uses the generic sparse-vmemmap population code.
Runtime intermediate page tables allocated by that code do not run a
page-table constructor. Freeing one during memory hot-remove therefore
runs a destructor without a matching constructor and corrupts
NR_PAGETABLE accounting.

Use PageTable() to run the destructor only for page-table pages whose
constructor initialized them. This keeps the arm64-created page-table
lifecycle balanced while safely freeing constructor-free vmemmap tables.

Fixes: c594b83457cc ("arm64: mm: call pagetable dtor when freeing hot-removed page tables")
Cc: stable at vger.kernel.org
Assisted-by: LLM
Signed-off-by: Muchun Song <songmuchun at bytedance.com>
---
 arch/arm64/mm/mmu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 7343ac9294f8..688b33095651 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1495,7 +1495,8 @@ static void free_hotplug_page_range(struct page *page, size_t size,
 
 static void free_hotplug_pgtable_page(struct page *page)
 {
-	pagetable_dtor(page_ptdesc(page));
+	if (PageTable(page))
+		pagetable_dtor(page_ptdesc(page));
 	free_hotplug_page_range(page, PAGE_SIZE, NULL);
 }
 
-- 
2.54.0