Hi List,
I have downloaded a kernel -  kernel-2.4.18.tar.bz2 and UML patch uml-patch-2.4.18-27.bz2. To compile and build UML, gave following commands:
1. Made a directory 'uml' in /usr/src.
    cd /usr/src/uml
    tar -xzvf linux-2.4.18.tar.bz2
2. cd /usr/src/uml/linux
   cat uml-patch-2.4.18-27.bz2 | bunzip2 - | patch -p1
3. make xconfig ARCH=um
4. make dep ARCH=um
5. make linux ARCH=um

The result is a file 'linux' created in top level source directory. After that I run it with root_fs as a file root_fs_toms1.7.205.bz2 with following command:
    ./linux ubd0=root_fs_toms1.7.205.
This boots UML. 
Now for accessing the host's files in UML, I gave following command:
         mount none /mnt/host -t ext2.
But it's giving error that: There is no special device 'none'.
Hostfs(ext2) is configured in UML as evident from /proc/filesystems in UML.
cat /proc/filesystems
    nodev   rootfs
    nodev   bdev
    nodev   proc
    nodev   sockfs
    nodev   tmpfs
    nodev   pipefs
    nodev   binfmt_misc
    nodev   devfs
   nodev   devpts
Is the things done correctly or I am missing something?