powerpc/mm: Move 64-bit unmapped_area to top of address space

powerpc/mm: Move 64-bit unmapped_area to top of address space

We currently place mmaps just below the stack on 32bit, but leave them
in the middle of the address space on 64bit:

00100000-00120000 r-xp 00100000 00:00 0                    [vdso]
10000000-10010000 r-xp 00000000 08:06 179534               /tmp/sleep
10010000-10020000 rw-p 00000000 08:06 179534               /tmp/sleep
10020000-10130000 rw-p 10020000 00:00 0                    [heap]
40000000000-40000030000 r-xp 00000000 08:06 440743         /lib64/ld-2.9.so
40000030000-40000040000 rw-p 00020000 08:06 440743         /lib64/ld-2.9.so
40000050000-400001f0000 r-xp 00000000 08:06 440671         /lib64/libc-2.9.so
400001f0000-40000200000 r--p 00190000 08:06 440671         /lib64/libc-2.9.so
40000200000-40000220000 rw-p 001a0000 08:06 440671         /lib64/libc-2.9.so
40000220000-40008230000 rw-p 40000220000 00:00 0
fffffbc0000-fffffd10000 rw-p fffffeb0000 00:00 0           [stack]

Right now it isn't an issue, but at some stage we will run into mmap or
hugetlb allocation issues. Using the same layout as 32bit gives us a
some breathing room. This matches what x86-64 is doing too.

00100000-00103000 r-xp 00100000 00:00 0                    [vdso]
10000000-10001000 r-xp 00000000 08:06 554894               /tmp/test
10010000-10011000 r--p 00000000 08:06 554894               /tmp/test
10011000-10012000 rw-p 00001000 08:06 554894               /tmp/test
10012000-10113000 rw-p 10012000 00:00 0                    [heap]
fffefdf7000-ffff7df8000 rw-p fffefdf7000 00:00 0
ffff7df8000-ffff7f97000 r-xp 00000000 08:06 130591         /lib64/libc-2.9.so
ffff7f97000-ffff7fa6000 ---p 0019f000 08:06 130591         /lib64/libc-2.9.so
ffff7fa6000-ffff7faa000 r--p 0019e000 08:06 130591         /lib64/libc-2.9.so
ffff7faa000-ffff7fc0000 rw-p 001a2000 08:06 130591         /lib64/libc-2.9.so
ffff7fc0000-ffff7fc4000 rw-p ffff7fc0000 00:00 0
ffff7fc4000-ffff7fec000 r-xp 00000000 08:06 130663         /lib64/ld-2.9.so
ffff7fee000-ffff7ff0000 rw-p ffff7fee000 00:00 0
ffff7ffa000-ffff7ffb000 rw-p ffff7ffa000 00:00 0
ffff7ffb000-ffff7ffc000 r--p 00027000 08:06 130663         /lib64/ld-2.9.so
ffff7ffc000-ffff7fff000 rw-p 00028000 08:06 130663         /lib64/ld-2.9.so
ffff7fff000-ffff8000000 rw-p ffff7fff000 00:00 0
fffffc59000-fffffc6e000 rw-p ffffffeb000 00:00 0           [stack]

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>