[PATCH v2 3/5] mm: Convert writeback BUG to WARN_ON

[Matthew Wilcox <willy@infradead.org>]

From: "Matthew Wilcox (Oracle)" <willy@infradead.org>

If this BUG() ever triggers, we'll have a dead system with no particular
information.  Dumping the page will give us a fighting chance of debugging
the problem, and I think it's safe for us to just continue if we try
to clear the writeback bit on a page which already has the writeback
bit clear.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/filemap.c        | 4 +---
 mm/page-writeback.c | 5 +++++
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index b7c5d2402370..401b24d980ba 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1293,9 +1293,7 @@ void end_page_writeback(struct page *page)
 		rotate_reclaimable_page(page);
 	}
 
-	if (!test_clear_page_writeback(page))
-		BUG();
-
+	test_clear_page_writeback(page);
 	smp_mb__after_atomic();
 	wake_up_page(page, PG_writeback);
 }
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7326b54ab728..ebaf0d8263a6 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2718,6 +2718,11 @@ int test_clear_page_writeback(struct page *page)
 	struct lruvec *lruvec;
 	int ret;
 
+	if (WARN_ON(!PageWriteback(page))) {
+		dump_page(page, "!writeback");
+		return false;
+	}
+
 	memcg = lock_page_memcg(page);
 	lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
 	if (mapping && mapping_use_writeback_tags(mapping)) {
-- 
2.25.1