After barebox up, I erased the /dev/nand0.rootfs.bb using erase command. kernel is already present in the /dev/nand0.kernel.
After ubiattach command, if I try the boot I am getting the below error. 
Actually my aim is to boot the kernel and it should mount to ubifs, to simplify the problem and reproduce the same I am just trying the boot after ubiattach instead of preparing the ubi0 device.
Barebox >/ubiattach /dev/nand0.rootfs
UBI: attaching mtd0 to ubi0
UBI: physical eraseblock size:   1048576 bytes (1024 KiB)
UBI: logical eraseblock size:    1044480 bytes
UBI: smallest flash I/O unit:    4096
UBI: sub-page size:              1024
UBI: VID header offset:          1024 (aligned 1024)
UBI: data offset:                4096
UBI: empty MTD device detected
UBI: create volume table (copy #1)
UBI: create volume table (copy #2)
registering /dev/ubi0
UBI: attached mtd0 to ubi0
UBI: MTD device name:            "nand0.rootfs"
UBI: MTD device size:            16 MiB
UBI: number of good PEBs:        16
UBI: number of bad PEBs:         0
UBI: max. allowed volumes:       128
UBI: wear-leveling threshold:    4096
UBI: number of internal volumes: 1
UBI: number of user volumes:     0
UBI: available PEBs:             10
UBI: total number of reserved PEBs: 6
UBI: number of PEBs reserved for bad PEB handling: 2
UBI: max/mean erase counter: 1/0

Barebox >/ boot
booting kernel of type uimage from /dev/nand0.kernel.bb
bootm /dev/nand0.kernel.bb
requesting xmalloc size: 2456544
ERROR: out of memory

Please somebody suggest me some pointer to solve this issue.

Thanks,
Madhu