Re: [PATCH v2 2/2] md: enable io polling

Andrzej Jakowski
2020-02-12T21:00:13Z

On 2/11/20 2:13 PM, Keith Busch wrote:
> I must be missing something: md's make_request_fn always returns
> BLK_QC_T_NONE for the cookie, and that couldn't get past blk_poll's
> blk_qc_t_valid(cookie) check. How does the initial blk_poll() caller get
> a valid cookie for an md backing device's request_queue? And how is the
> same cookie valid for more than one request_queue?

That's true md_make_request() always returns BLK_QC_T_NONE. md_make_request()
recursively calls generic_make_request() for the underlying device (e.g. nvme).
That block io request directed to member disk is added into bio_list and is
processed later by top level generic_make_request(). generic_make_request()
returns cookie that is returned by blk_mq_make_request().
That cookie is later used to poll for completion.