[PATCH 1/9] mm: Always initialise folio->_deferred_list

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

For compound pages which are at least order-2 (and hence have a
deferred_list), initialise it and then we can check at free that the
page is not part of a deferred list.  We recently found this useful to
rule out a source of corruption.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/huge_memory.c | 2 --
 mm/hugetlb.c     | 3 ++-
 mm/internal.h    | 2 ++
 mm/memcontrol.c  | 2 ++
 mm/page_alloc.c  | 9 +++++----
 5 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9859aa4f7553..04fb994a7b0b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -792,8 +792,6 @@ void folio_prep_large_rmappable(struct folio *folio)
 {
 	if (!folio || !folio_test_large(folio))
 		return;
-	if (folio_order(folio) > 1)
-		INIT_LIST_HEAD(&folio->_deferred_list);
 	folio_set_large_rmappable(folio);
 }
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 23ef240ba48a..7e9a766059aa 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1796,7 +1796,8 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
 		destroy_compound_gigantic_folio(folio, huge_page_order(h));
 		free_gigantic_folio(folio, huge_page_order(h));
 	} else {
-		__free_pages(&folio->page, huge_page_order(h));
+		INIT_LIST_HEAD(&folio->_deferred_list);
+		folio_put(folio);
 	}
 }
 
diff --git a/mm/internal.h b/mm/internal.h
index 7e486f2c502c..10895ec52546 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -525,6 +525,8 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
 	atomic_set(&folio->_entire_mapcount, -1);
 	atomic_set(&folio->_nr_pages_mapped, 0);
 	atomic_set(&folio->_pincount, 0);
+	if (order > 1)
+		INIT_LIST_HEAD(&folio->_deferred_list);
 }
 
 static inline void prep_compound_tail(struct page *head, int tail_idx)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fabce2b50c69..a2a74d4ca0b1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7448,6 +7448,8 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 	struct obj_cgroup *objcg;
 
 	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
+	VM_BUG_ON_FOLIO(folio_order(folio) > 1 &&
+			!list_empty(&folio->_deferred_list), folio);
 
 	/*
 	 * Nobody should be changing or seriously looking at
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 14d39f34d336..4301146a5bf4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1006,10 +1006,11 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 		}
 		break;
 	case 2:
-		/*
-		 * the second tail page: ->mapping is
-		 * deferred_list.next -- ignore value.
-		 */
+		/* the second tail page: deferred_list overlaps ->mapping */
+		if (unlikely(!list_empty(&folio->_deferred_list))) {
+			bad_page(page, "on deferred list");
+			goto out;
+		}
 		break;
 	default:
 		if (page->mapping != TAIL_MAPPING) {
-- 
2.43.0