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

Keith Busch
2020-02-12T21:42:15Z

On Wed, Feb 12, 2020 at 02:00:10PM -0700, Andrzej Jakowski wrote:
> 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.

Okay, that's a nice subtlety. But it means the original caller gets the
cookie from the last submission in the chain. If md recieves a single
request that has to be split among more than one member disk, the cookie
you're using to control the polling is valid only for one of the
request_queue's and may break others.