Hi,
The 4 level pagetable code changed the exit_mmap code to rely on
TASK_SIZE. On some architectures (eg ppc64 and ia64), this is a per task
property and bad things can happen in certain circumstances when using
it.
It is possible for one task to end up "owning" an mm from another - we
have seen this with the procfs code when process 1 accesses
/proc/pid/cmdline of process 2 while it is exiting. Process 2 exits
but does not tear its mm down. Later on process 1 finishes with the proc
file and the mm gets torn down at this point.
Now if process 1 was 32bit and process 2 was 64bit then we end up using
a bad value for TASK_SIZE in exit_mmap. We only tear down part of the
address space and leave half initialised pagetables and entries in the
MMU etc.
MM_VM_SIZE() was created for this purpose (and is used in the next line
for tlb_finish_mmu), so use it. I moved the PGD round up of TASK_SIZE
into the default MM_VM_SIZE.
As an aside, all architectures except one define FIRST_USER_PGD_NR as 0:
include/asm-arm26/pgtable.h:#define FIRST_USER_PGD_NR 1
It would be nice to get rid of one more magic constant and just clear
from 0 ... MM_VM_SIZE(). That would make it consistent with the
tlb_flush_mmu call below it too.