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

Andrzej Jakowski
2020-02-13T20:19:45Z

On 2/12/20 2:42 PM, Keith Busch wrote:
> 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.

Correct, I agree that it is an issue. I can see at least two ways how to solve it:
1. Provide a mechanism in md accounting for outstanding IOs, storing cookie information
for them. md_poll() will then use valid cookie's
2. Provide similar mechanism abstracted for stackable block devices and block layer could
handle completions for subordinate bios in an abstracted way in blk_poll() routine.
How do you Guys see this going?