powerpc: ppc64 address space capped at 32TB, mmap randomisation disabled

Commit fba2369 (mm: use vm_unmapped_area() on powerpc architecture)
has a bug in slice_scan_available() where we compare an unsigned long
(high_slices) against a shifted int. As a result, comparisons against
the top 32 bits of high_slices (representing the top 32TB) always
returns 0 and the top of our mmap region is clamped at 32TB

This also breaks mmap randomisation since the randomised address is
always up near the top of the address space and it gets clamped down
to 32TB.

Cc: [email protected] # v3.10+
Signed-off-by: Anton Blanchard <[email protected]>
Acked-by: Michel Lespinasse <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>

arch/powerpc/mm/slice.c

@@ -258,7 +258,7 @@ static bool slice_scan_available(unsigned long addr,
 		slice = GET_HIGH_SLICE_INDEX(addr);
 		*boundary_addr = (slice + end) ?
 			((slice + end) << SLICE_HIGH_SHIFT) : SLICE_LOW_TOP;
-		return !!(available.high_slices & (1u << slice));
+		return !!(available.high_slices & (1ul << slice));
 	}
 }