[PATCH] mm: Assert the folio is locked in folio_start_writeback()

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

The folio must be locked when we start writeback in order to
prevent writeback from being started twice on the same folio.
I don't expect this to catch any problems, but it should be
good documentation.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 mm/page-writeback.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index eb55ece39c56..8b325aa525eb 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -3109,6 +3109,7 @@ void __folio_start_writeback(struct folio *folio, bool keep_write)
 	int access_ret;
 
 	VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
+	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 
 	if (mapping && mapping_use_writeback_tags(mapping)) {
 		XA_STATE(xas, &mapping->i_pages, folio_index(folio));
-- 
2.47.2