Add the --enable-mremap option.

Add the --enable-mremap option, and disable the use of mremap(2) by
default, for the same reason that freeing chunks via munmap(2) is
disabled by default on Linux: semi-permanent VM map fragmentation.

include/jemalloc/internal/jemalloc_internal.h.in

@@ -123,6 +123,13 @@ static const bool config_prof_libunwind =
     false
 #endif
     ;
+static const bool config_mremap =
+#ifdef JEMALLOC_MREMAP
+    true
+#else
+    false
+#endif
+    ;
 static const bool config_munmap =
 #ifdef JEMALLOC_MUNMAP
     true

include/jemalloc/jemalloc_defs.h.in

@@ -193,12 +193,18 @@
 
 /*
  * If defined, use munmap() to unmap freed chunks, rather than storing them for
- * later reuse.  This is automatically disabled if configuration determines
- * that common sequences of mmap()/munmap() calls will cause virtual memory map
- * holes.
+ * later reuse.  This is disabled by default on Linux because common sequences
+ * of mmap()/munmap() calls will cause virtual memory map holes.
  */
 #undef JEMALLOC_MUNMAP
 
+/*
+ * If defined, use mremap(...MREMAP_FIXED...) for huge realloc().  This is
+ * disabled by default because it is Linux-specific and it will cause virtual
+ * memory map holes, much like munmap(2) does.
+ */
+#undef JEMALLOC_MREMAP
+
 /* TLS is used to map arenas and magazine caches to threads. */
 #undef JEMALLOC_TLS
 
@@ -221,9 +227,6 @@
 #undef JEMALLOC_ZONE
 #undef JEMALLOC_ZONE_VERSION
 
-/* If defined, use mremap(...MREMAP_FIXED...) for huge realloc(). */
-#undef JEMALLOC_MREMAP_FIXED
-
 /*
  * Methods for purging unused pages differ between operating systems.
  *