ext2 cannot be compiled as module

[Ronghua Zhang]

Hi,
  I compiled ext2 support into a module, but when I mount an ext
partition, kernel oops. The following is what I have done:
 1. copy ext2 directory into another one, say test
 2. change the file system name into 'test'
 3. compile it into a module ext2.o
 4. insmod ext2.o
 5. mound /dev/hdb1 /mnt -t test, kernel oops.

I found out the oops happens at ext2_read_inode() called by
ext2_read_super() to read inode for the root. ext2_read_inode() need to
access inode->i_sb->u.ext2_sb.s_es, but suprisingly, this s_es is NULL!
I further found that this is because inode->i_sb does NOT points to the
supper block whose content is filled by ext2_read_super(), instead it
points to other unknown memory. Printing out inode->s_sb in
ext2_read_inode() and priniting out sb before ext2_read_super() executes
sb->s_root = d_alloc_root(iget(sb, EXT2_ROOT_INO)) can easily reveal this
problem.

I also wrote a very simple file system, only implementing read_super and
read_inode. When it's compiled as a module, the same problem exists. But
when it's compiled into the kernel, no problem at all!

Is it a bug or am I missing something? BTW, the kernel is 2.4.19 and gcc
is 3.2

Ronghua