Man page 9 : blk_queue_merge_bvec

blk_queue_merge_bvec - set a merge_bvec function for queue

void blk_queue_merge_bvec(struct request_queue *  q , merge_bvec_fn *  mbfn );

q
    queue

mbfn
    merge_bvec_fn

Usually queues have static limitations on the max sectors or segments that we can put in a request. Stacking drivers may have some settings that are dynamic, and thus we have to query the queue whether it is ok to add a new bio_vec to a bio at a given offset or not. If the block device has such limitations, it needs to register a merge_bvec_fn to control the size of bioAqs sent to it. Note that a block device *must* allow a single page to be added to an empty bio. The block device driver may want to use the bio_split function to deal with these bioAqs. By default no merge_bvec_fn is defined for a queue, and only the fixed limits are honored.