generic_make_request does not return any status. The success/failure status of the request, along with notification of completion, is delivered asynchronously through the bh->b_end_io function described (one day) else where.
The caller of generic_make_request must make sure that b_page, b_addr, b_size are set to describe the memory buffer, that b_rdev and b_rsector are set to describe the device address, and the b_end_io and optionally b_private are set to describe how completion notification should be signaled. BH_Mapped should also be set (to confirm that b_dev and b_blocknr are valid).
generic_make_request and the drivers it calls may use b_reqnext, and may change b_rdev and b_rsector. So the values of these fields should NOT be depended on after the call to generic_make_request. Because of this, the caller should record the device address information in b_dev and b_blocknr.
Apart from those fields mentioned above, no other fields, and in particular, no other flags, are changed by generic_make_request or any lower level drivers.