mm/ksm: document smart scan mode

This adds documentation for the smart scan mode of KSM.

[akpm@linux-foundation.org: fix typo]
[akpm@linux-foundation.org: document that smart_scan defaults to on]
Link: https://lkml.kernel.org/r/20230926040939.516161-4-shr@devkernel.ioSigned-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/ksm.rst

@@ -155,6 +155,15 @@ stable_node_chains_prune_millisecs
         scan. It's a noop if not a single KSM page hit the
         ``max_page_sharing`` yet.
 
+smart_scan
+        Historically KSM checked every candidate page for each scan. It did
+        not take into account historic information.  When smart scan is
+        enabled, pages that have previously not been de-duplicated get
+        skipped. How often these pages are skipped depends on how often
+        de-duplication has already been tried and failed. By default this
+        optimization is enabled.  The ``pages_skipped`` metric shows how
+        effective the setting is.
+
 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
 general_profit