Re: [PATCH 4/4] ipc/shm.c: Increase the defaults for SHMALL, SHMMAX.

[Davidlohr Bueso]

On Mon, 2014-04-21 at 16:26 +0200, Manfred Spraul wrote:
> System V shared memory
> 
> a) can be abused to trigger out-of-memory conditions and the standard
>    measures against out-of-memory do not work:
> 
>     - it is not possible to use setrlimit to limit the size of shm segments.
> 
>     - segments can exist without association with any processes, thus
>       the oom-killer is unable to free that memory.
> 
> b) is typically used for shared information - today often multiple GB.
>    (e.g. database shared buffers)
> 
> The current default is a maximum segment size of 32 MB and a maximum total
> size of 8 GB. This is often too much for a) and not enough for b), which
> means that lots of users must change the defaults.

Per Andrew's request, I think the following should go here from the
changelog of my patch:

Unix has historically required setting these limits for shared
memory, and Linux inherited such behavior. The consequence of this
is added complexity for users and administrators. One very common
example are Database setup/installation documents and scripts,
where users must manually calculate the values for these limits.
This also requires (some) knowledge of how the underlying memory
management works, thus causing, in many occasions, the limits to
just be flat out wrong. Disabling these limits sooner could have
saved companies a lot of time, headaches and money for support.
But it's never too late, simplify users life now.


> This patch increases the default limits (nearly) to the maximum, which is
> perfect for case b). The defaults are used after boot and as the initial
> value for each new namespace.
> 
> Admins/distros that need a protection against a) should reduce the limits
> and/or enable shm_rmid_forced.
> 
> Further notes:
> - The patch only changes default, overrides behave as before:
>         # sysctl kernel.shmall=33554432
>   would recreate the previous limit for SHMMAX (for the current namespace).
> 
> - Disabling sysv shm allocation is possible with:
>         # sysctl kernel.shmall=0
>   (not a new feature, also per-namespace)
> 
> - The limits are intentionally set to a value slightly less than ULONG_MAX,
>   to avoid triggering overflows in user space apps.
>   [not unreasonable, see http://marc.info/?l=linux-mm&m=139638334330127]
> 
> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> Reported-by: Davidlohr Bueso <davidlohr@xxxxxx>
> Cc: mtk.manpages@xxxxxxxxx
> ---
>  include/linux/shm.h      | 3 +--
>  include/uapi/linux/shm.h | 8 +++-----
>  2 files changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/shm.h b/include/linux/shm.h
> index 1e2cd2e..57d7770 100644
> --- a/include/linux/shm.h
> +++ b/include/linux/shm.h
> @@ -3,9 +3,8 @@
>  
>  #include <asm/page.h>
>  #include <uapi/linux/shm.h>
> -
> -#define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */
>  #include <asm/shmparam.h>
> +
>  struct shmid_kernel /* private to the kernel */
>  {	
>  	struct kern_ipc_perm	shm_perm;
> diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
> index 78b6941..74e786d 100644
> --- a/include/uapi/linux/shm.h
> +++ b/include/uapi/linux/shm.h
> @@ -9,15 +9,13 @@
>  
>  /*
>   * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can
> - * be increased by sysctl
> + * be modified by sysctl.
>   */
>  
> -#define SHMMAX 0x2000000		 /* max shared seg size (bytes) */
>  #define SHMMIN 1			 /* min shared seg size (bytes) */
>  #define SHMMNI 4096			 /* max num of segs system wide */
> -#ifndef __KERNEL__
> -#define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16))
> -#endif
> +#define SHMMAX (ULONG_MAX - (1L<<24))	 /* max shared seg size (bytes) */
> +#define SHMALL (ULONG_MAX - (1L<<24))	 /* max shm system wide (pages) */
>  #define SHMSEG SHMMNI			 /* max shared segs per process */
>  
> 


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