Re: [RFC PATCH 1/2] blk-mq: don't call callbacks for requests that bypassed the scheduler

On Fri, Aug 27, 2021 at 12:41:31PM +0000, Niklas Cassel wrote:

From: Niklas Cassel <redacted>

Currently, __blk_mq_alloc_request() calls ops.prepare_request and sets
RQF_ELVPRIV.

Therefore, (if the request is not a flush) the RQF_ELVPRIV flag will be
set for the request in blk_mq_submit_bio(), regardless if the request
was submitted to a scheduler, or bypassed the scheduler.

Later, blk_mq_free_request() checks if the RQF_ELVPRIV flag is set,
if it is, the ops.finish_request callback will be called.

The problem with this is that the finish_request scheduler callback
will be called for requests that bypassed the scheduler.

Fix this by calling the scheduler ops.prepare_request callback, and
set the RQF_ELVPRIV flag only immediately before calling the insert
callback.

One request could be inserted more than one times, such as requeue,
however __blk_mq_alloc_request() is just run once, so is it fine to
call ->prepare_request more than one time for same request?

Or I am wondering why not call ->prepare_request when the following
check is true?

	if (e && e->type->ops.prepare_request && !op_is_flush(data->cmd_flags) &&
		!blk_op_is_passthrough(data->cmd_flags))
		e->type->ops.prepare_request()




Thanks, 
Ming