Re: dm-mpath: return DM_MAPIO_REQUEUE in case of rq allocation failure

[Ming Lei <ming.lei@redhat.com>]

On Mon, Sep 25, 2017 at 12:17:03PM -0400, Mike Snitzer wrote:
> On Mon, Sep 25 2017 at 12:10pm -0400,
> Ming Lei <ming.lei@redhat.com> wrote:
> 
> > On Mon, Sep 25, 2017 at 03:23:16PM +0000, Bart Van Assche wrote:
> > > On Mon, 2017-09-25 at 11:06 +0800, Ming Lei wrote:
> > > > On Fri, Sep 22, 2017 at 05:54:48PM +0000, Bart Van Assche wrote:
> > > > > On Sat, 2017-09-23 at 01:44 +0800, Ming Lei wrote:
> > > > > > On Fri, Sep 22, 2017 at 03:06:16PM +0000, Bart Van Assche wrote:
> > > > > > > On Fri, 2017-09-22 at 09:35 +0800, Ming Lei wrote:
> > > > > > > > +		/*
> > > > > > > > +		 * blk-mq's SCHED_RESTART can cover this requeue, so
> > > > > > > > +		 * we needn't to deal with it by DELAY_REQUEUE. More
> > > > > > > > +		 * importantly, we have to return DM_MAPIO_REQUEUE
> > > > > > > > +		 * so that blk-mq can get the queue busy feedback,
> > > > > > > > +		 * otherwise I/O merge can be hurt.
> > > > > > > > +		 */
> > > > > > > > +		if (q->mq_ops)
> > > > > > > > +			return DM_MAPIO_REQUEUE;
> > > > > > > > +		else
> > > > > > > > +			return DM_MAPIO_DELAY_REQUEUE;
> > > > > > > >  	}
> > > > > > > 
> > > > > > > This patch is inferior to what I posted because this patch does not avoid
> > > > > > > the delay if multiple LUNs are associated with the same SCSI host. Consider
> > > > > > > e.g. the following configuration:
> > > > > > > * A single SCSI host with two SCSI LUNs associated to that host, e.g. /dev/sda
> > > > > > >   and /dev/sdb.
> > > > > > > * A dm-mpath instance has been created on top of /dev/sda.
> > > > > > > If all tags are in use by requests queued to /dev/sdb, no dm requests are in
> > > > > > > progress and a request is submitted against the dm-mpath device then the
> > > > > > > blk_get_request(q, GFP_ATOMIC) call will fail. The request will be requeued
> > > > > > > and the queue will be rerun after a delay.
> > > > > > > 
> > > > > > > My patch does not introduce a delay in this case.
> > > > > > 
> > > > > > That delay may not matter because SCHED_RESTART will run queue just
> > > > > > after one request is completed.
> > > > > 
> > > > > Did you understand what I wrote? SCHED_RESTART will be set for /dev/sdb but not
> > > > > for the dm queue. That's what I was trying to explain to you in my previous e-mail.
> > > > 
> > > > The patch I posted in this thread will set SCHED_RESTART for dm queue.
> > > 
> > > This is not how communication on an open source mailing list is assumed to work.
> > > If you know that you are wrong you are assumed either to shut up or to admit it.
> 
> Code speaks much better than unnecessarily caustic exchanges.  Not sure
> why Bart persists with that.. but that's for him to sort out.
>  
> > You just mentioned 'This patch is inferior' and never explained my patch
> > is wrong, so please go ahead and show me why this patch(the post in this
> > thread, also the following link) is wrong.
> > 
> > 	https://marc.info/?l=linux-block&m=150604412910113&w=2
> > 
> > I admit both are two ways for the issue, but I don't think my patch
> > is wrong. Your approach can be a very big change because .queue_rq()
> > will block, and I also mentioned it might cause AIO regression.
> 
> I have no interest in changing DM multipath to block in .queue_rq()
> So please consider that approach a dead end.
> 
> Ming, just iterate on your revised patchset, test and post when you're
> happy with it.

Hi Mike,

If you don't consider to change mpath into block in .queue_rq() now,
please take this patch first, and I am sure this way is correct, and
even it can be thought as a fix.


Thanks,
Ming