linux-kernel - Re: [PATCH v1 3/3] docs: Add information about ipc sysctls limitations

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Date:   Mon, 19 Sep 2022 10:29:02 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Alexey Gladkov <legion@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux.dev>,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Manfred Spraul <manfred@...orfullife.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v1 3/3] docs: Add information about ipc sysctls limitations

Alexey Gladkov <legion@...nel.org> writes:

> 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.

Agreed. Documenting these limits only apply to their ipc namespace is
overdue.

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

> Signed-off-by: Alexey Gladkov <legion@...nel.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
>  ======