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From:   Alexey Gladkov <legion@kernel.org>
To:     LKML <linux-kernel@vger.kernel.org>,
        Linux Containers <containers@lists.linux.dev>,
        linux-doc@vger.kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <brauner@kernel.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Manfred Spraul <manfred@colorfullife.com>,
        Jonathan Corbet <corbet@lwn.net>
Subject: [PATCH v1 3/3] docs: Add information about ipc sysctls limitations
Date:   Tue, 16 Aug 2022 17:42:45 +0200
Message-Id: <9b566b9b7313300d60b7dfd785ab041bed757ae0.1660664258.git.legion@kernel.org>
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After 25b21cb2f6d6 ("[PATCH] IPC namespace core") and 4e9823111bdc
("[PATCH] IPC namespace - shm") the shared memory page count stopped
being global and started counting per ipc namespace. The documentation
and shmget(2) still says that shmall is a global option.

shmget(2):

SHMALL System-wide limit on the total amount of shared memory, measured
in units of the system page size. On Linux, this limit can be read and
modified via /proc/sys/kernel/shmall.

I think the changes made in 2006 should be documented.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
 Documentation/admin-guide/sysctl/kernel.rst | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index ddccd1077462..9ad344b5e7a1 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -541,6 +541,9 @@ default (``MSGMNB``).
 ``msgmni`` is the maximum number of IPC queues. 32000 by default
 (``MSGMNI``).
 
+All of these parameters are set per ipc namespace. The maximum number of bytes
+in POSIX message queues is limited by ``RLIMIT_MSGQUEUE``. This limit is
+respected hierarchically in the each user namespace.
 
 msg_next_id, sem_next_id, and shm_next_id (System V IPC)
 ========================================================
@@ -1169,15 +1172,20 @@ are doing anyway :)
 shmall
 ======
 
-This parameter sets the total amount of shared memory pages that
-can be used system wide. Hence, ``shmall`` should always be at least
-``ceil(shmmax/PAGE_SIZE)``.
+This parameter sets the total amount of shared memory pages that can be used
+inside ipc namespace. The shared memory pages counting occurs for each ipc
+namespace separately and is not inherited. Hence, ``shmall`` should always be at
+least ``ceil(shmmax/PAGE_SIZE)``.
 
 If you are not sure what the default ``PAGE_SIZE`` is on your Linux
 system, you can run the following command::
 
 	# getconf PAGE_SIZE
 
+To reduce or disable the ability to allocate shared memory, you must create a
+new ipc namespace, set this parameter to the required value and prohibit the
+creation of a new ipc namespace in the current user namespace or cgroups can
+be used.
 
 shmmax
 ======
-- 
2.33.4