Man page 9 : blk_add_request_payload

blk_add_request_payload - add a payload to a request

void blk_add_request_payload(struct request *  rq , struct page *  page , unsigned int  len );

rq
    request to update

page
    page backing the payload

len
    length of the payload.

This allows to later add a payload to an already submitted request by a block driver. The driver needs to take care of freeing the payload itself.

Note that this is a quite horrible hack and nothing but handling of discard requests should ever use it.