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<chapter id="mm">

  <?dbhtml filename="mm.html"?>

  <title>Memory management</title>

  <section>

    <title>Virtual memory management</title>

    <section>

      <title>Address spaces</title>

      <para></para>

    </section>

    <section>

      <title>Virtual address translation</title>

      <para></para>

    </section>

  </section>

  <section>

    <title>Physical memory management</title>

    <section id="zones_and_frames">

      <title>Zones and frames</title>

      <para>Physical memory is divided into zones. Each zone represents

      continuous area of physical memory frames. Allocation of frames is

      handled by the <link linkend="buddy_allocator">buddy allocator</link>

      associated with the zone. Zone also contains information about free and

      occupied frames and its base addresss in the memory. Some of the

      architectures (Mips, PPC) have only one zone, that covers whole physical

      memory. Other architectures (IA32) have multiple zones.</para>

    </section>

    <section id="buddy_allocator">

      <title>Buddy allocator</title>

      <section>

        <title>Overview</title>

        <para>Physical memory allocation inside one <link

        linkend="zones_and_frames">memory zone</link> is being handled by

        buddy allocation system. This approach greatly reduces possibility of

        outer memory fragmentation and helps in allocating bigger continious

        blocks of physical memory aligned to their size. Problem of inner

        memory fragmentation is being solved by <link linkend="slab">SLAB

        allocation system.</link></para>

        <graphic fileref="images/mm1.png" />

        <para>Frames are grouped into bigger blocks and blocks of the size

        <mathphrase>2<superscript>i</superscript></mathphrase> are stored in

        the list indexed with <varname>i</varname> (so called order index). If

        list contains 2 ajacent blocks (of a same size of cause) they can be

        merged into the bigger one and moved into the list with higher order

        index, thus making possible allocation of a bigger block.</para>

      </section>

      <section>

        <title>Implementation</title>

        <formalpara>

          <title>Data organization</title>

          <para>Buddy allocator always uses first frame to represent frame

          block. This frame contains <varname>buddy_order</varname> variable

          to provide information about the block size it actually represents (

          <mathphrase>2<superscript>buddy_order</superscript></mathphrase>

          frames block). Other frames in block have this value set to magic

          <constant>BUDDY_INNER_BLOCK</constant> that is much greater than

          buddy <varname>max_order</varname> value.</para>

          <para>Each <varname>frame_t</varname> also contains pointer member

          to hold frame structure in the linked list inside one order.</para>

        </formalpara>

        <formalpara>

          <title>Allocation algorithm</title>

          <para>Upon <mathphrase>2<superscript>i</superscript></mathphrase>

          frames block allocation request, allocator checks if there are any

          blocks available at the order list <varname>i</varname>. If yes,

          removes block from order list and returns its address. If no,

          recursively allocates

          <mathphrase>2<superscript>i+1</superscript></mathphrase> frame

          block, splits it into two

          <mathphrase>2<superscript>i</superscript></mathphrase> frame blocks.

          Then adds one of the blocks to the <varname>i</varname> order list

          and returns address of another.</para>

        </formalpara>

        <formalpara>

          <title>Deallocation algorithm</title>

          <para>Check if block has so called buddy (another free

          <mathphrase>2<superscript>i</superscript></mathphrase> frame block

          that can be linked with freed block into the

          <mathphrase>2<superscript>i+1</superscript></mathphrase> block).

          Technically, buddy is a odd/even block for even/odd block

          respectively. Plus we can put an extra requirement, that resulting

          block must be aligned to its size. This requirement guarantees

          natural block alignment for the blocks coming out the allocation

          system.

      </para>

      <para>

      Using direct pointer arithmetics, <varname>frame_t::ref_count</varname> and <varname>frame_t::buddy_order</varname> variables,

      finding buddy is done at constant time.

      </para>

        </formalpara>

      </section>

    </section>

    <section id="slab">

      <title>Slab allocator</title>

      <para>Kernel memory allocation is handled by slab.</para>

    </section>

    <section>

      <title>Memory sharing</title>

      <para>Not implemented yet(?)</para>

    </section>

  </section>

</chapter>