+ * @tree: tree so we can call our merge_bio hook
+ * @wbc: optional writeback control for io accounting
+ * @page: page to add to the bio
+ * @pg_offset: offset of the new bio or to check whether we are adding
+ * a contiguous page to the previous one
+ * @size: portion of page that we want to write
+ * @offset: starting offset in the page
+ * @bdev: attach newly created bios to this bdev
+ * @bio_ret: must be valid pointer, newly allocated bio will be stored there
+ * @end_io_func: end_io callback for new bio
+ * @mirror_num: desired mirror to read/write
+ * @prev_bio_flags: flags of previous bio to see if we can merge the current one
+ * @bio_flags: flags of the current bio to see if we can merge them