Memory can be laid out with large gaps on 64-bit PowerPC, so switch to

VM_PHYSSEG_SPARSE.

sys/powerpc/include/vmparam.h

@@ -140,9 +140,14 @@ struct pmap_physseg {
 #define	VM_PHYSSEG_MAX		16	/* 1? */
 
 /*
- * The physical address space is densely populated.
+ * The physical address space is densely populated on 32-bit systems,
+ * but may not be on 64-bit ones.
  */
+#ifdef __powerpc64__
+#define	VM_PHYSSEG_SPARSE
+#else
 #define	VM_PHYSSEG_DENSE
+#endif
 
 /*
  * Create three free page pools: VM_FREEPOOL_DEFAULT is the default pool