Re: [dm-devel] Barriers still not passing on simple dm devices...

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Mar 24 2009, Mikulas Patocka wrote:
> 
> 
> On Mon, 23 Mar 2009, Eric Sandeen wrote:
> 
> > I've noticed that on 2.6.29-rcX, with Andi's patch
> > (ab4c1424882be9cd70b89abf2b484add355712fa, dm: support barriers on
> > simple devices) barriers are still getting rejected on these simple devices.
> > 
> > The problem is in __generic_make_request():
> > 
> >                 if (bio_barrier(bio) && bio_has_data(bio) &&
> >                     (q->next_ordered == QUEUE_ORDERED_NONE)) {
> >                         err = -EOPNOTSUPP;
> >                         goto end_io;
> >                 }
> > 
> > and dm isn't flagging its queue as supporting ordered writes, so it's
> > rejected here.
> > 
> > Doing something like this:
> > 
> > + if (t->barriers_supported)
> > +         blk_queue_ordered(q, QUEUE_ORDERED_DRAIN, NULL);
> > 
> > somewhere in dm (I stuck it in dm_table_set_restrictions() - almost
> > certainly the wrong thing to do) did get my dm-linear device to mount
> > with xfs, w/o xfs complaining that its mount-time barrier tests failed.
> > 
> > So what's the right way around this?  What should dm (or md for that
> > matter) advertise on their queues about ordered-ness?  Should there be
> > some sort of "QUEUE_ORDERED_PASSTHROUGH" or something to say "this level
> > doesn't care, ask the next level" or somesuch?  Or should it inherit the
> > flag from the next level down?  Ideas?
> > 
> > Thanks,
> > -Eric
> > 
> > --
> > dm-devel mailing list
> > dm-devel@redhat.com
> > https://www.redhat.com/mailman/listinfo/dm-devel
> 
> Hi
> 
> This is misdesign in generic bio layer and it should be fixed there. I 
> think it is blocking barrier support in md-raid1 too. Jens, pls apply the 
> attached patch.
> 
> Mikulas
> 
> ----
> 
> Move test for not-supported barriers to __make_request.
> 
> This test prevents barriers from being dispatched to device mapper
> and md.
> 
> This test is sensible only for drivers that use requests (such as disk
> drivers), not for drivers that use bios.
> 
> It is better to fix it in generic code than to make workaround for it
> in device mapper and md.

So you audited any ->make_request_fn style driver and made sure they
rejected barriers?

> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  block/blk-core.c |   11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> Index: linux-2.6.29-rc6-devel/block/blk-core.c
> ===================================================================
> --- linux-2.6.29-rc6-devel.orig/block/blk-core.c	2009-02-23 18:43:37.000000000 +0100
> +++ linux-2.6.29-rc6-devel/block/blk-core.c	2009-02-23 18:44:27.000000000 +0100
> @@ -1145,6 +1145,12 @@ static int __make_request(struct request
>  	const int unplug = bio_unplug(bio);
>  	int rw_flags;
>  
> +	if (bio_barrier(bio) && bio_has_data(bio) &&
> +	    (q->next_ordered == QUEUE_ORDERED_NONE)) {
> +		bio_endio(bio, -EOPNOTSUPP);
> +		return 0;
> +	}
> +
>  	nr_sectors = bio_sectors(bio);
>  
>  	/*
> @@ -1450,11 +1456,6 @@ static inline void __generic_make_reques
>  			err = -EOPNOTSUPP;
>  			goto end_io;
>  		}
> -		if (bio_barrier(bio) && bio_has_data(bio) &&
> -		    (q->next_ordered == QUEUE_ORDERED_NONE)) {
> -			err = -EOPNOTSUPP;
> -			goto end_io;
> -		}
>  
>  		ret = q->make_request_fn(q, bio);
>  	} while (ret);
> > 

-- 
Jens Axboe