Re: block: fix direct dispatch issue failure for clones

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Dec 06 2018 at  8:04pm -0500,
jianchao.wang <jianchao.w.wang@oracle.com> wrote:

> 
> 
> On 12/7/18 6:20 AM, Jens Axboe wrote:
> > After the direct dispatch corruption fix, we permanently disallow direct
> > dispatch of non read/write requests. This works fine off the normal IO
> > path, as they will be retried like any other failed direct dispatch
> > request. But for the blk_insert_cloned_request() that only DM uses to
> > bypass the bottom level scheduler, we always first attempt direct
> > dispatch. For some types of requests, that's now a permanent failure,
> > and no amount of retrying will make that succeed.
> > 
> > Don't use direct dispatch off the cloned insert path, always just use
> > bypass inserts. This still bypasses the bottom level scheduler, which is
> > what DM wants.
> > 
> > Fixes: ffe81d45322c ("blk-mq: fix corruption with direct issue")
> > Signed-off-by: Jens Axboe <axboe@kernel.dk>
> > 
> > ---
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index deb56932f8c4..4c44e6fa0d08 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -2637,7 +2637,8 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
> >  		 * bypass a potential scheduler on the bottom device for
> >  		 * insert.
> >  		 */
> > -		return blk_mq_request_issue_directly(rq);
> > +		blk_mq_request_bypass_insert(rq, true);
> > +		return BLK_STS_OK;
> >  	}
> >  
> >  	spin_lock_irqsave(q->queue_lock, flags);
> > 
> Not sure about this because it will break the merging promotion for request based DM
> from Ming.
> 396eaf21ee17c476e8f66249fb1f4a39003d0ab4
> (blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback)

Ngh.. yeah, forgot about that convoluted feedback loop.

> We could use some other way to fix this.

Yeah, afraid we have to.