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



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 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:

[  752.301388] drbd0: Writing meta data super block now.
[  752.349713] drbd0: local disk flush failed with status -5
[  752.416256] drbd0: local disk flush failed with status -5
[  753.419254] drbd0: local disk flush failed with status -5
[  753.925726] drbd0: local disk flush failed with status -5
[  754.551176] drbd0: local disk flush failed with status -5
[  754.806052] drbd0: local disk flush failed with status -5
[  755.327988] drbd0: local disk flush failed with status -5
[  755.781863] drbd0: local disk flush failed with status -5
[  756.266694] drbd0: local disk flush failed with status -5





Anders

> diff --git a/block/blk-barrier.c b/block/blk-barrier.c
> index 6901eed..55c5f1f 100644
> --- a/block/blk-barrier.c
> +++ b/block/blk-barrier.c
> @@ -259,8 +259,11 @@ int blk_do_ordered(struct request_queue *q, struct request **rqp)
>  
>  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);
>  }
> @@ -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
> 
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