bug#35167: About chroot some question on centos6 kernel:

[往事随风]

OS centos6.10
kernel vmlinuz-2.6.32-754.el6.x86_64
hello!
grub-install  in a new disk /mnt/boot;copy /bin/bash and *.so ; chroot 
/mnt/sysroot is ok!exit and ctrl+d
use the new disk startup,
"dracut warning can't mount root filesystemmount :/dev/sda3 already mounted or 
/sysroot busy
mount: according to mtab, /dev/sdb3 is already mounted on /mnt/sysroot"
don't chroot /mnt/sysroot 
startup ——success

why?!  I don't now! 

thank you