Btrfs: add tracepoint for em's EEXIST case

This is adding a tracepoint 'btrfs_handle_em_exist' to help debug the
subtle bugs around merge_extent_mapping.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -551,6 +551,9 @@ int btrfs_add_extent_mapping(struct extent_map_tree *em_tree,
 		ret = 0;
 
 		existing = search_extent_mapping(em_tree, start, len);
+
+		trace_btrfs_handle_em_exist(existing, em, start, len);
+
 		/*
 		 * existing will always be non-NULL, since there must be
 		 * extent causing the -EEXIST.

include/trace/events/btrfs.h

@@ -248,6 +248,41 @@ TRACE_EVENT_CONDITION(btrfs_get_extent,
 		  __entry->refs, __entry->compress_type)
 );
 
+TRACE_EVENT(btrfs_handle_em_exist,
+
+	TP_PROTO(const struct extent_map *existing, const struct extent_map *map, u64 start, u64 len),
+
+	TP_ARGS(existing, map, start, len),
+
+	TP_STRUCT__entry(
+		__field(	u64,  e_start		)
+		__field(	u64,  e_len		)
+		__field(	u64,  map_start		)
+		__field(	u64,  map_len		)
+		__field(	u64,  start		)
+		__field(	u64,  len		)
+	),
+
+	TP_fast_assign(
+		__entry->e_start	= existing->start;
+		__entry->e_len		= existing->len;
+		__entry->map_start	= map->start;
+		__entry->map_len	= map->len;
+		__entry->start		= start;
+		__entry->len		= len;
+	),
+
+	TP_printk("start=%llu len=%llu "
+		  "existing(start=%llu len=%llu) "
+		  "em(start=%llu len=%llu)",
+		  (unsigned long long)__entry->start,
+		  (unsigned long long)__entry->len,
+		  (unsigned long long)__entry->e_start,
+		  (unsigned long long)__entry->e_len,
+		  (unsigned long long)__entry->map_start,
+		  (unsigned long long)__entry->map_len)
+);
+
 /* file extent item */
 DECLARE_EVENT_CLASS(btrfs__file_extent_item_regular,