Re: [PATCH RESEND] Btrfs: fix wrong write offset when replacing a device

[Josef Bacik <jbacik@fusionio.com>]

On Thu, Jul 04, 2013 at 04:14:23PM +0200, Stefan Behrens wrote:
> Miao Xie reported the following issue:
> 
> The filesystem was corrupted after we did a device replace.
> 
> Steps to reproduce:
>  # mkfs.btrfs -f -m single -d raid10 <device0>..<device3>
>  # mount <device0> <mnt>
>  # btrfs replace start -rfB 1 <device4> <mnt>
>  # umount <mnt>
>  # btrfsck <device4>
> 
> The reason for the issue is that we changed the write offset by mistake,
> introduced by commit 625f1c8dc.
> 
> We read the data from the source device at first, and then write the
> data into the corresponding place of the new device. In order to
> implement the "-r" option, the source location is remapped using
> btrfs_map_block(). The read takes place on the mapped location, and
> the write needs to take place on the unmapped location. Currently
> the write is using the mapped location, and this commit changes it
> back by undoing the change to the write address that the aforementioned
> commit added by mistake.
> 

I'd like to see a xfstest for this please so we can make sure something like
this doesn't happen in the future.  You can use SCRATCH_DEV_POOL to have
multiple devices.  Please cc linux-btrfs when you submit it so I can review it
and hopefully get it in faster.  Thanks,

Josef