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docs: add information about ipc sysctls limitations

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After 25b21cb2f6 ("[PATCH] IPC namespace core") and 4e9823111b
("[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.

Link: https://lkml.kernel.org/r/09e99911071766958af488beb4e8a728a4f12135.1705333426.git.legion@kernel.org
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lkml.kernel.org/r/ede20ddf7be48b93e8084c3be2e920841ee1a641.1663756794.git.legion@kernel.org
Cc: Christian Brauner <brauner@kernel.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Joel Granados <joel.granados@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Alexey Gladkov 2024-01-15 15:46:42 +00:00 committed by Andrew Morton
parent 50ec499b9a
commit 9220066ea8
1 changed files with 11 additions and 3 deletions
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14
Documentation/admin-guide/sysctl/kernel.rst
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@ -594,6 +594,9 @@ 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)
========================================================
@ -1274,15 +1277,20 @@ 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
======