--- x/include/linux/rmap.h
+++ y/include/linux/rmap.h
@@ -327,6 +327,7 @@ static __always_inline void __folio_dup_
 
 	switch (level) {
 	case RMAP_LEVEL_PTE:
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			atomic_inc(&page->_mapcount);
 			break;
@@ -419,6 +420,7 @@ static __always_inline int __folio_try_d
 					return -EBUSY;
 		}
 
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			if (PageAnonExclusive(page))
 				ClearPageAnonExclusive(page);
--- x/mm/rmap.c
+++ y/mm/rmap.c
@@ -1149,6 +1149,7 @@ static __always_inline unsigned int __fo
 
 	switch (level) {
 	case RMAP_LEVEL_PTE:
+		BUG_ON(atomic_read(&page->_mapcount) < -1);
 		if (!folio_test_large(folio)) {
 			nr = atomic_inc_and_test(&page->_mapcount);
 			break;
@@ -1503,6 +1504,7 @@ static __always_inline void __folio_remo
 
 	switch (level) {
 	case RMAP_LEVEL_PTE:
+		BUG_ON(atomic_read(&page->_mapcount) < 0);
 		if (!folio_test_large(folio)) {
 			nr = atomic_add_negative(-1, &page->_mapcount);
 			break;