fs: remove default_backing_dev_info

Now that default_backing_dev_info is not used for writeback purposes we can
git rid of it easily:

 - instead of using it's name for tracing unregistered bdi we just use
   "unknown"
 - btrfs and ceph can just assign the default read ahead window themselves
   like several other filesystems already do.
 - we can assign noop_backing_dev_info as the default one in alloc_super.
   All filesystems already either assigned their own or
   noop_backing_dev_info.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 74f5207bd090..0e9310905413 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -156,10 +156,8 @@ DECLARE_EVENT_CLASS(writeback_work_class,
                 __field(int, reason)
         ),
         TP_fast_assign(
-                struct device *dev = bdi->dev;
-                if (!dev)
-                        dev = default_backing_dev_info.dev;
-                strncpy(__entry->name, dev_name(dev), 32);
+                strncpy(__entry->name,
+                        bdi->dev ? dev_name(bdi->dev) : "(unknown)", 32);
                 __entry->nr_pages = work->nr_pages;
                 __entry->sb_dev = work->sb ? work->sb->s_dev : 0;
                 __entry->sync_mode = work->sync_mode;