[PATCH] writeback: verify that underlying dev exists before getting its name

[Sasha Levin]

We can't assume there is always an underlying device and must verify that
as well. Otherwise, we'd deref a NULL ptr here.

Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
 include/trace/events/writeback.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index fff846b..8f9efaa 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -66,7 +66,9 @@ TRACE_EVENT(writeback_dirty_page,
 
 	TP_fast_assign(
 		strncpy(__entry->name,
-			mapping ? dev_name(inode_to_bdi(mapping->host)->dev) : "(unknown)", 32);
+			mapping && inode_to_bdi(mapping->host)->dev ?
+				dev_name(inode_to_bdi(mapping->host)->dev) :
+				"(unknown)", 32);
 		__entry->ino = mapping ? mapping->host->i_ino : 0;
 		__entry->index = page->index;
 	),
-- 
1.7.10.4

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