Bad things can happen if a 32-bit process is the last user of a 64-bit mm. 
TASK_SIZE isn't a constant, and we can end up clearing page tables only up
to the 32-bit TASK_SIZE instead of all the way.  We should probably
double-check every instance of TASK_SIZE or USER_PTRS_PER_PGD for this kind
of problem.

We should also double-check that MM_VM_SIZE() and other such things are
correctly defined on all architectures.  I already fixed ppc64 which let it
stay as TASK_SIZE, and hence dependent on the _current_ context instead of
the mm in the argument.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>