[PATCH v1 11/11] mm/page_alloc: check page->private upon page free

[Zi Yan]

Any user is supposed to zero their allocated page->private at free time,
including both head and tail pages if the freed page is compound. Add
VM_WARN_ON_ONCE to check page->private.

Whoever encountering warnings from this commit needs to fix code that
does not zero ->private when freeing a page/folio.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Brendan Jackman <jackmanb@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
---
 mm/page_alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d88c8c67ac0b..d8b7f36e9364 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1410,6 +1410,7 @@ __always_inline bool __free_pages_prepare(struct page *page,
 				}
 			}
 			(page + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
+			VM_WARN_ON_ONCE((page + i)->private);
 		}
 	}
 	if (folio_test_anon(folio)) {
@@ -1429,7 +1430,7 @@ __always_inline bool __free_pages_prepare(struct page *page,
 
 	page_cpupid_reset_last(page);
 	page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
-	page->private = 0;
+	VM_WARN_ON_ONCE(page->private);
 	reset_page_owner(page, order);
 	page_table_check_free(page, order);
 	pgalloc_tag_sub(page, 1 << order);
-- 
2.51.0