请高手解答：request-buffer与request中的bio是什么关系？

最近看cowloop-3.0的源代码，对request有一些疑问，
在cowloop.c中的cowlo_do_request函数中对request处理时，直接把request中的buffer做为请求中的数据，
这个req->buffer与bio是什么关系，如何把bio中的数据映射到req->buffer中去的？

static long int
cowlo_do_request(struct request *req)
{
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len = req->current_nr_sectors << 9;
offset = (loff_t) req->sector << 9;

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switch (rq_data_dir(req)) {
/**********************************************************/
case READ:
switch ( cowlo_checkio(cowdev, len, offset) ) {
case ALLCOW:
rv = cowlo_readcow(cowdev, req->buffer, len, offset);
break;

case ALLRDO:
rv = cowlo_readrdo(cowdev, req->buffer, len, offset);
break;

case MIXEDUP:
rv = cowlo_readmix(cowdev, req->buffer, len, offset);
break;

default:
rv = 0; /* never happens */
}
break;
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return (rv <= 0 ? 0 : 1);
}