Try to merge with currents plugged list
bool blk_attempt_plug_merge(struct request_queue * q, struct bio * bio, unsigned int * request_count);
q
request_queue new bio is being queued at
bio
new bio being queued
request_count
out parameter for number of traversed plugged requests
Determine whether bio being queued on q can be merged with a request on current's plugged list. Returns true if merge was successful, otherwise false.
Plugging coalesces IOs from the same issuer for the same purpose without going through q->queue_lock. As such it's more of an issuing mechanism than scheduling, and the request, while may have elvpriv data, is not added on the elevator at this point. In addition, we don't have reliable access to the elevator outside queue lock. Only check basic merging parameters without querying the elevator.
Caller must ensure !blk_queue_nomerges(q) beforehand.