docs/mm: extend ksm doc

This adds a description of the new prctl interface for KSM and also adds a
general section on security concerns.

Link: https://lkml.kernel.org/r/20230314204557.3863923-1-shr@devkernel.io
Signed-off-by: Stefan Roesch <shr@devkernel.io>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index 6eece5671ab643014e1d999588d18e6490a5be19..60dc42b3a6a808a1aaa44f21499da0af89eac71d 100644 (file)
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -20,13 +20,15 @@ content which can be replaced by a single write-protected page (which
 is automatically copied if a process later wants to update its
 content). The amount of pages that KSM daemon scans in a single pass
 and the time between the passes are configured using :ref:`sysfs
-intraface <ksm_sysfs>`
+interface <ksm_sysfs>`
 
 KSM only merges anonymous (private) pages, never pagecache (file) pages.
 KSM's merged pages were originally locked into kernel memory, but can now
 be swapped out just like other user pages (but sharing is broken when they
 are swapped back in: ksmd must rediscover their identity and merge again).
 
+.. _ksm_madvise:
+
 Controlling KSM with madvise
 ============================
 
@@ -68,6 +70,43 @@ Applications should be considerate in their use of MADV_MERGEABLE,
 restricting its use to areas likely to benefit.  KSM's scans may use a lot
 of processing power: some installations will disable KSM for that reason.
 
+Controlling KSM with prctl
+============================
+
+KSM can be enabled for a process or a cgroup, by using the prctl(2) system
+call::
+
+       int prctl(PR_SET_MEMORY_MERGE, 1)
+
+The app may call
+
+::
+
+       int prctl(PR_SET_MEMORY_MERGE, 0)
+
+to cancel that advice and restore unshared pages: whereupon KSM
+unmerges whatever is merged for that process.  Note: this unmerging call
+may suddenly require more memory than is available - possibly failing
+with EAGAIN, but more probably arousing the Out-Of-Memory killer.
+
+The restrictions mentioned in :ref:`Controlling KSM with madvise <ksm_madvise>`'
+also apply here. Also consider the security implications of using KSM.
+
+KSM security concerns
+=======================
+
+KSM has the possibility of memory side channel attacks. When individual
+VMA's have KSM enabled, the security aspect needs to be considered.
+
+An individual workload doesn't know what else is running on
+the machine, so it needs to be highly conservative about what it can
+give up for system-wide merging.
+
+However, if the system is dedicated to running multiple jobs within the
+same security domain, there is a usecase where multiple instances of the
+same job are running inside a safe shared security domain and using the
+same sensitive data.
+
 .. _ksm_sysfs:
 
 KSM daemon sysfs interface