[PATCH v2 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() detect constructor-backed pages and run the
destructor before releasing their memory to fix the problem.

Fixes: 49f599666420 ("mm: call ctor/dtor for kernel PTEs")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
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 5cca90fc1df6..771a7d60b0a5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3969,23 +3969,6 @@ 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);
@@ -4078,6 +4061,26 @@ 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