Re: [syzbot] [nilfs?] WARNING in nilfs_btree_assign (3)

[Ryusuke Konishi]

Isolate whether the ongoing patch protecting the referenced buffer
head affects reproducibility of the issue.

#syz test

diff --git a/fs/nilfs2/page.c b/fs/nilfs2/page.c
index 9de2a494a069..899686d2e5f7 100644
--- a/fs/nilfs2/page.c
+++ b/fs/nilfs2/page.c
@@ -392,6 +392,11 @@ void nilfs_clear_dirty_pages(struct address_space *mapping)
 /**
  * nilfs_clear_folio_dirty - discard dirty folio
  * @folio: dirty folio that will be discarded
+ *
+ * nilfs_clear_folio_dirty() clears working states including dirty state for
+ * the folio and its buffers.  If the folio has buffers, clear only if it is
+ * confirmed that none of the buffer heads are busy (none have valid
+ * references and none are locked).
  */
 void nilfs_clear_folio_dirty(struct folio *folio)
 {
@@ -399,10 +404,6 @@ void nilfs_clear_folio_dirty(struct folio *folio)
 
 	BUG_ON(!folio_test_locked(folio));
 
-	folio_clear_uptodate(folio);
-	folio_clear_mappedtodisk(folio);
-	folio_clear_checked(folio);
-
 	head = folio_buffers(folio);
 	if (head) {
 		const unsigned long clear_bits =
@@ -410,6 +411,25 @@ void nilfs_clear_folio_dirty(struct folio *folio)
 			 BIT(BH_Async_Write) | BIT(BH_NILFS_Volatile) |
 			 BIT(BH_NILFS_Checked) | BIT(BH_NILFS_Redirected) |
 			 BIT(BH_Delay));
+		bool busy, invalidated = false;
+
+recheck_buffers:
+		busy = false;
+		bh = head;
+		do {
+			if (atomic_read(&bh->b_count) | buffer_locked(bh)) {
+				busy = true;
+				break;
+			}
+		} while (bh = bh->b_this_page, bh != head);
+
+		if (busy) {
+			if (invalidated)
+				return;
+			invalidate_bh_lrus();
+			invalidated = true;
+			goto recheck_buffers;
+		}
 
 		bh = head;
 		do {
@@ -419,6 +439,9 @@ void nilfs_clear_folio_dirty(struct folio *folio)
 		} while (bh = bh->b_this_page, bh != head);
 	}
 
+	folio_clear_uptodate(folio);
+	folio_clear_mappedtodisk(folio);
+	folio_clear_checked(folio);
 	__nilfs_clear_folio_dirty(folio);
 }
 
-- 
2.43.0