The existing comment provides a good overview of KSM implementation. Let's
update it to reflect recent additions of "chain" and "dup" variants of the
stable tree nodes and mark it as "DOC:" for inclusion into the KSM
documentation.
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
---
mm/ksm.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 58c2741..54155b1 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -51,7 +51,9 @@
#define DO_NUMA(x) do { } while (0)
#endif
-/*
+/**
+ * DOC: Overview
+ *
* A few notes about the KSM scanning process,
* to make it easier to understand the data structures below:
*
@@ -67,6 +69,21 @@
* this tree is fully assured to be working (except when pages are unmapped),
* and therefore this tree is called the stable tree.
*
+ * The stable tree node includes information required for reverse
+ * mapping from a KSM page to virtual addresses that map this page.
+ *
+ * In order to avoid large latencies of the rmap walks on KSM pages,
+ * KSM maintains two types of nodes in the stable tree:
+ *
+ * * the regular nodes that keep the reverse mapping structures in a
+ * linked list
+ * * the "chains" that link nodes ("dups") that represent the same
+ * write protected memory content, but each "dup" corresponds to a
+ * different KSM page copy of that content
+ *
+ * Internally, the regular nodes, "dups" and "chains" are represented
+ * using the same :c:type:`struct stable_node` structure.
+ *
* In addition to the stable tree, KSM uses a second data structure called the
* unstable tree: this tree holds pointers to pages which have been found to
* be "unchanged for a period of time". The unstable tree sorts these pages
--
2.7.4
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