On 04/19/2017 01:45 AM, Christoph Hellwig wrote:

On Fri, Apr 14, 2017 at 07:02:54AM -0500, Goldwyn Rodrigues wrote:

quoted

From: Goldwyn Rodrigues <redacted>

quoted

+	/* Request queue supports BIO_NOWAIT */
+	queue_flag_set_unlocked(QUEUE_FLAG_NOWAIT, q);

BIO_NOWAIT is gone.  And the comment would not be needed if the
flag had a more descriptive name, e.g. QUEUE_FLAG_NOWAIT_SUPPORT.

And I think all request based drivers should set the flag implicitly
as ->queuecommand can't sleep, and ->queue_rq only when it's always
offloaded to a workqueue when the BLK_MQ_F_BLOCKING flag is set.

We introduced QUEUE_FLAG_NOWAIT for devices which would not wait for
request completions. The ones which wait are MD devices because of sync
or suspend operations.

The only user of BLK_MQ_F_NONBLOCKING seems to be nbd. As you mentioned,
it uses the flag to offload it to a workqueue.

The other way to do it implicitly is to change the flag to
BLK_MAY_BLOCK_REQS and use it for devices which do wait such as md/dm.
Is that what you are hinting at? Or do you have something else in mind?


-- 
Goldwyn