[dm-devel] [PATCH] dm-mirror: do not degrade the mirror on discard error

[James Bottomley]

On Thu, 2015-02-12 at 10:09 -0500, Mikulas Patocka wrote:
> It may be possible that a device claims discard support but it rejects
> discards with -EOPNOTSUPP. It happens when using loopback on ext2/ext3
> filesystem driven by the ext4 driver. It may also happen if the underlying
> devices are moved from one disk on another.
> 
> If discard error happens, we reject the bio with -EOPNOTSUPP, but we do
> not degrade the array.
> 
> This patch fixes failed test shell/lvconvert-repair-transient.sh in the
> lvm2 testsuite if the testsuite is extracted on an ext2 or ext3 filesystem
> and it is being driven by the ext4 driver.
> 
> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> 
> Index: linux-2.6/drivers/md/dm-raid1.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-raid1.c
> +++ linux-2.6/drivers/md/dm-raid1.c
> @@ -604,6 +604,15 @@ static void write_callback(unsigned long
>  		return;
>  	}
>  
> +	/*
> +	 * If the bio is discard, return an error, but do not
> +	 * degrade the array.
> +	 */
> +	if (bio->bi_rw & REQ_DISCARD) {
> +		bio_endio(bio, -EOPNOTSUPP);

I think the error gets ignored, so this is probably harmless, but
shouldn't you propagate the actual error here?  discard is advisory and
can fail for a variety of reasons (alignment being chief) for which
EOPNOTSUPP is inappropriate.

James