[PATCH v3 1/4] mm: fix missing page-table destructor in pagetable_free()

[Muchun Song]

Since commit 49f599666420 ("mm: call ctor/dtor for kernel PTEs"),
pte_alloc_one_kernel() runs the page-table constructor for kernel PTE
tables.

HVO (HugeTLB Vmemmap Optimization) uses pte_alloc_one_kernel() when
splitting a PMD. Restoring the vmemmap backing pages does not collapse
the PTE table, so a later memory hot-remove eventually frees it through
pagetable_free() on x86-64. That releases the page without running the
matching destructor and leaves NR_PAGETABLE elevated.

Make pagetable_free() the common free-side primitive for page-table
pages. It checks PageTable() and runs the destructor only when the
constructor state is still present. Callers that explicitly ran
pagetable_dtor() have already cleared PageTable(), so they are not
destructed twice; pages that never ran a constructor are freed directly.

Fixes: 49f599666420 ("mm: call ctor/dtor for kernel PTEs")
Cc: stable at vger.kernel.org # bf9e4e30f353: x86/mm: use pagetable_free()
Cc: stable at vger.kernel.org # 39406c05f8f1: x86/mm: Fix freeing of PMD-sized vmemmap pages
Cc: stable at vger.kernel.org
Assisted-by: LLM
Signed-off-by: Muchun Song <songmuchun at bytedance.com>
Reviewed-by: Jose A. Perez de Azpillaga <azpijr at gmail.com>
Reviewed-by: Kevin Brodsky <kevin.brodsky at arm.com>
Acked-by: Dave Hansen <dave.hansen at intel.com>
Acked-by: David Hildenbrand (Arm) <david at kernel.org>
---
v3:
- Explain why pagetable_free() does not run the destructor twice
  (requested by Kevin Brodsky)
- Add x86 stable prerequisites (suggested by
  Jose A. Perez de Azpillaga)
- Collect Acked-by from Dave Hansen and David Hildenbrand
- Collect Reviewed-by from Jose A. Perez de Azpillaga and Kevin
  Brodsky

v2:
- Move destructor handling from x86 into pagetable_free() (suggested by
  David Hildenbrand)
- Trim and update the commit message (requested by Dave Hansen)
---
 include/linux/mm.h | 37 ++++++++++++++++++++-----------------
 1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd09c438fa23..466875c5908b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3812,23 +3812,6 @@ static inline void pagetable_free_kernel(struct ptdesc *pt)
 	__pagetable_free(pt);
 }
 #endif
-/**
- * pagetable_free - Free pagetables
- * @pt:	The page table descriptor
- *
- * pagetable_free frees the memory of all page tables described by a page
- * table descriptor and the memory for the descriptor itself.
- */
-static inline void pagetable_free(struct ptdesc *pt)
-{
-	if (ptdesc_test_kernel(pt)) {
-		ptdesc_clear_kernel(pt);
-		pagetable_free_kernel(pt);
-	} else {
-		__pagetable_free(pt);
-	}
-}
-
 #if defined(CONFIG_SPLIT_PTE_PTLOCKS)
 #if ALLOC_SPLIT_PTLOCKS
 void __init ptlock_cache_init(void);
@@ -3921,6 +3904,26 @@ static inline void pagetable_dtor(struct ptdesc *ptdesc)
 	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
 }
 
+/**
+ * pagetable_free - Free pagetables
+ * @pt:	The page table descriptor
+ *
+ * pagetable_free frees the memory of all page tables described by a page
+ * table descriptor and the memory for the descriptor itself.
+ */
+static inline void pagetable_free(struct ptdesc *pt)
+{
+	if (PageTable(ptdesc_page(pt)))
+		pagetable_dtor(pt);
+
+	if (ptdesc_test_kernel(pt)) {
+		ptdesc_clear_kernel(pt);
+		pagetable_free_kernel(pt);
+	} else {
+		__pagetable_free(pt);
+	}
+}
+
 static inline void pagetable_dtor_free(struct ptdesc *ptdesc)
 {
 	pagetable_dtor(ptdesc);
-- 
2.54.0