docs/vm: ksm: add "Design" section

the ksmd CPU usage and possibly slowdown the readonly computations on

the KSM pages of the applications.

Design

======

Overview

--------

.. kernel-doc:: mm/ksm.c

   :DOC: Overview

Reverse mapping

---------------

KSM maintains reverse mapping information for KSM pages in the stable

tree.

If a KSM page is shared between less than ``max_page_sharing`` VMAs,

the node of the stable tree that represents such KSM page points to a

list of :c:type:`struct rmap_item` and the ``page->mapping`` of the

KSM page points to the stable tree node.

When the sharing passes this threshold, KSM adds a second dimension to

the stable tree. The tree node becomes a "chain" that links one or

more "dups". Each "dup" keeps reverse mapping information for a KSM

page with ``page->mapping`` pointing to that "dup".

Every "chain" and all "dups" linked into a "chain" enforce the

invariant that they represent the same write protected memory content,

even if each "dup" will be pointed by a different KSM page copy of

that content.

This way the stable tree lookup computational complexity is unaffected

if compared to an unlimited list of reverse mappings. It is still

enforced that there cannot be KSM page content duplicates in the

stable tree itself.

Reference

---------

.. kernel-doc:: mm/ksm.c

   :functions: mm_slot ksm_scan stable_node rmap_item

--

Izik Eidus,

Hugh Dickins, 17 Nov 2009