Add opt.thp which allows explicit hugepage usage.

"always" marks all user mappings as MADV_HUGEPAGE; while "never" marks all
mappings as MADV_NOHUGEPAGE. The default setting "default" does not change any
settings.  Note that all the madvise calls are part of the default extent hooks
by design, so that customized extent hooks have complete control over the
mappings including hugepage settings.

doc/jemalloc.xml.in

@@ -1217,6 +1217,28 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         default maximum is 32 KiB (2^15).</para></listitem>
       </varlistentry>
 
+      <varlistentry id="opt.thp">
+        <term>
+          <mallctl>opt.thp</mallctl>
+          (<type>const char *</type>)
+          <literal>r-</literal>
+        </term>
+        <listitem><para>Transparent hugepage (THP) mode. Settings "always",
+        "never" and "default" are available if THP is supported by the operating
+        system.  The "always" setting enables transparent hugepage for all user
+        memory mappings with
+        <parameter><constant>MADV_HUGEPAGE</constant></parameter>; "never"
+        ensures no transparent hugepage with
+        <parameter><constant>MADV_NOHUGEPAGE</constant></parameter>; the default
+        setting "default" makes no changes.  Note that: this option does not
+        affect THP for jemalloc internal metadata (see <link
+        linkend="opt.metadata_thp"><mallctl>opt.metadata_thp</mallctl></link>);
+        in addition, for arenas with customized <link
+        linkend="arena.i.extent_hooks"><mallctl>extent_hooks</mallctl></link>,
+        this option is bypassed as it is implemented as part of the default
+        extent hooks.</para></listitem>
+      </varlistentry>
+
       <varlistentry id="opt.prof">
         <term>
           <mallctl>opt.prof</mallctl>