problem with quota on rootfs

[Joerg Maier <joerg.maier@xxxxxxxxxxxxxxxxxx>]

Hello,

i'd like to introduce quota to a 2 years old xfs root filesystem. my 
kernel and my quotatools are quite recent:
kernel : 2.6.8.1
quota-3.10
i build quota and xfs hard into the kernel:
CONFIG_XFS_FS=y
# CONFIG_XFS_RT is not set
CONFIG_XFS_QUOTA=y
# CONFIG_XFS_SECURITY is not set
# CONFIG_XFS_POSIX_ACL is not set
CONFIG_QUOTA=y
CONFIG_QUOTACTL=y
# CONFIG_VXFS_FS is not set

when i try to remount my / fs with usrquota as an option it does not work:
# mount -t xfs -o remount,usrquota /dev/sda6 /
mount: / not mounted already, or bad option

i tried to use quota and xfs on that system on a loop filesystem and it 
worked fine.

Could anybody help me how to get quota work on my xfs rootfs? I don't 
know how to see which version of quota is on the device as well, 
xfs_info shows nothing that gives me a hint.

thanks in spe,
joerg maier