[Ubuntu-PH] how to mount a fielsystem in LVM

[cj pangilinan]

i have a reiserfs in my LVM. i don't know how to mount it. i have to
mount my / to access /etc/sudoers..

 i usermod my only account in ubuntu and i haven't set my root passwd.
so i can't log in to the system..
i boot from the livecd but i dont know hot to mount my / dir. its LVM. 
what is the device of the LVM in the /dev dir?

pls help..
thanks..

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