Add arena.i.retain_grow_limit

This option controls the max size when grow_retained.  This is useful when we
have customized extent hooks reserving physical memory (e.g. 1G huge pages).
Without this feature, the default increasing sequence could result in fragmented
and wasted physical memory.

doc/jemalloc.xml.in

@@ -1683,6 +1683,22 @@ malloc_conf = "xmalloc:true";]]></programlisting>
         for additional information.</para></listitem>
       </varlistentry>
 
+      <varlistentry id="arena.i.retain_grow_limit">
+        <term>
+          <mallctl>arena.&lt;i&gt;.retain_grow_limit</mallctl>
+          (<type>size_t</type>)
+          <literal>rw</literal>
+        </term>
+        <listitem><para>Maximum size to grow retained region (only relevant when
+        <link linkend="opt.retain"><mallctl>opt.retain</mallctl></link> is
+        enabled).  This controls the maximum increment to expand virtual memory,
+        or allocation through <link
+        linkend="arena.i.extent_hooks"><mallctl>arena.&lt;i&gt;extent_hooks</mallctl></link>.
+        In particular, if customized extent hooks reserve physical memory
+        (e.g. 1G huge pages), this is useful to control the allocation hook's
+        input size.  The default is no limit.</para></listitem>
+      </varlistentry>
+
       <varlistentry id="arena.i.extent_hooks">
         <term>
           <mallctl>arena.&lt;i&gt;.extent_hooks</mallctl>