Man page 9 : generic_pipe_buf_unmap

generic_pipe_buf_unmap - unmap a previously mapped pipe buffer

void generic_pipe_buf_unmap(struct pipe_inode_info *  pipe , struct pipe_buffer *  buf , void *  map_data );

pipe
    the pipe that the buffer belongs to

buf
    the buffer that should be unmapped

map_data
    the data that the mapping function returned

This function undoes the mapping that ->map provided.