[dm-devel] Re: device mapper not reporting no-barrier-support?

[Jens Axboe]

On Tue, Feb 26 2008, Anders Henke wrote:
> On Tue, Feb 26 2008 Jens Axboe wrote:
> > On Tue, Feb 26 2008, Alasdair G Kergon wrote:
> > > On Mon, Feb 25, 2008 at 03:20:50PM -0800, Andrew Morton wrote:
> > > > On Mon, 25 Feb 2008 14:26:15 +0100 Anders Henke <anders.henke at 1und1.de> wrote:
> > > > > I'm currently stuck between Kernel LVM and DRBD, as I'm using Kernel
> > > > > 2.6.24.2 with DRBD 8.2.5 on top of an LVM2 device (LV).
> > > > > -LVM2/device mapper doesn't support write barriers
> > > 
> > > That's right.
> > > 
> > > > > -DRBD uses blkdev_issue_flush() to flush its metadata to disk.
> > > 
> > > Which won't work if device-mapper is underneath.
> > > 
> > > > >  On a no-barrier-device, DRBD should receive EOPNOTSUPP, but
> > > > >  it really does receive an EIO. Promptly, DRBD gives the
> > > > >  error message "drbd0: local disk flush failed with status -5".
> > > > > I've posted a lengty summary of my findings to
> > > > > http://lists.linbit.com/pipermail/drbd-user/2008-February/008665.html
> > > > > ... that DRBD does catch the EOPNOTSUPP for blkdev_issue_flush and
> > > > > BIO_RW_BARRIER, but the lvm implementation of blkdev_issue_flush in
> > > > > 2.6.24.2 aparently does return EIO for blkdev_issue_flush.
> > > > I'd say it's a DM bug.
> > > 
> > > The dm code is unchanged, but look at the limited endio handling in
> > > ll_rw_blk.c:
> > > 
> > > static void bio_end_empty_barrier(struct bio *bio, int err)
> > > {
> > >         if (err)
> > >                 clear_bit(BIO_UPTODATE, &bio->bi_flags);
> > > 
> > >         complete(bio->bi_private);
> > > }
> > > 
> > > int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
> > > {
> > > ...
> > >         wait_for_completion(&wait);
> > >         if (error_sector)
> > >                 *error_sector = bio->bi_sector;
> > >         ret = 0;
> > >         if (!bio_flagged(bio, BIO_UPTODATE))
> > >                 ret = -EIO;
> > 
> > You are right, the return value got broken there. Does this make it
> > return -EOPNOTSUPP properly for you?
> 
> 
> No, it doesn't.
> 
> 
> 
> I've applied your patch manually, as 2.6.24.2. doesn't have a "blk-barrier.c":
> 
> ---cut
> --- linux-2.6.24.2/block/ll_rw_blk.c.prepatch   2008-02-11
> 06:51:11.000000000 +0100
> +++ linux-2.6.24.2/block/ll_rw_blk.c    2008-02-26 20:02:28.514641620
> +0100
> @@ -2667,8 +2667,11 @@
>  
>  static void bio_end_empty_barrier(struct bio *bio, int err)
>  {
> -       if (err)
> +       if (err) {
> +               if (err == -EOPNOTSUPP)
> +                       set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
>                 clear_bit(BIO_UPTODATE, &bio->bi_flags);
> +       }
>  
>         complete(bio->bi_private);
>  }
> ---cut
> 
> ... and the resulting kernel shows exactly the same behaviour than before:

Not surprising, as you missed half of the patch:

> > @@ -309,7 +312,9 @@ int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
> >  		*error_sector = bio->bi_sector;
> >  
> >  	ret = 0;
> > -	if (!bio_flagged(bio, BIO_UPTODATE))
> > +	if (bio_flagged(bio, BIO_EOPNOTSUPP))
> > +		ret = -EOPNOTSUPP;
> > +	else if (!bio_flagged(bio, BIO_UPTODATE))
> >  		ret = -EIO;
> >  
> >  	bio_put(bio);

-- 
Jens Axboe