Re: [PATCH v2] ipc,shm: disable shmmax and shmall by default

[Michael Kerrisk]

On Sat, Apr 12, 2014 at 5:22 AM, Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> From: Davidlohr Bueso <davidlohr@xxxxxx>
>
> The default size for shmmax is, and always has been, 32Mb.
> Today, in the XXI century, it seems that this value is rather small,
> making users have to increase it via sysctl, which can cause
> unnecessary work and userspace application workarounds[1].
>
> Instead of choosing yet another arbitrary value, larger than 32Mb,
> this patch disables the use of both shmmax and shmall by default,
> allowing users to create segments of unlimited sizes. Users and
> applications that already explicitly set these values through sysctl
> are left untouched, and thus does not change any of the behavior.
>
> So a value of 0 bytes or pages, for shmmax and shmall, respectively,
> implies unlimited memory, as opposed to disabling sysv shared memory.
> This is safe as 0 cannot possibly be used previously as SHMMIN is
> hardcoded to 1 and cannot be modified.
>
> This change allows Linux to treat shm just as regular anonymous memory.
> One important difference between them, though, is handling out-of-memory
> conditions: as opposed to regular anon memory, the OOM killer will not
> free the memory as it is shm, allowing users to potentially abuse this.
> To overcome this situation, the shm_rmid_forced option must be enabled.
>
> [1]: http://rhaas.blogspot.com/2012/06/absurd-shared-memory-limits.html
>
> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>

Of the two proposed approaches (the other being
marc.info/?l=linux-kernel&m=139730332306185), this looks preferable to
me, since it allows strange users to maintain historical behavior
(i.e., the ability to set a limit) if they really want it, so:

Acked-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>

One or two comments below, that you might consider for your v3 patch.

> ---
> Changes from v1:
>  - Respect SHMMIN even when shmmax is 0 (unlimited).
>    This fixes the shmget02 test that broke in v1. (per Manfred)
>
>  - Update changelog regarding OOM description (per Kosaki)
>
>  include/linux/shm.h      | 2 +-
>  include/uapi/linux/shm.h | 8 ++++----
>  ipc/shm.c                | 6 ++++--
>  3 files changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/shm.h b/include/linux/shm.h
> index 1e2cd2e..0ca06a3 100644
> --- a/include/linux/shm.h
> +++ b/include/linux/shm.h
> @@ -4,7 +4,7 @@
>  #include <asm/page.h>
>  #include <uapi/linux/shm.h>
>
> -#define SHMALL (SHMMAX/PAGE_SIZE*(SHMMNI/16)) /* max shm system wide (pages) */
> +#define SHMALL 0 /* max shm system wide (pages) */
>  #include <asm/shmparam.h>
>  struct shmid_kernel /* private to the kernel */
>  {
> diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
> index 78b6941..5f0ef28 100644
> --- a/include/uapi/linux/shm.h
> +++ b/include/uapi/linux/shm.h
> @@ -9,14 +9,14 @@
>
>  /*
>   * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can
> - * be increased by sysctl
> + * be increased by sysctl. By default, disable SHMMAX and SHMALL with

s/increased/modified/

> + * 0 bytes, thus allowing processes to have unlimited shared memory.
>   */
> -
> -#define SHMMAX 0x2000000                /* max shared seg size (bytes) */
> +#define SHMMAX 0                        /* max shared seg size (bytes) */

I suggest: s/(bytes)/(bytes); 0 means "no limit" */

>  #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))
> +#define SHMALL 0

As long as we're here, let's add a meaningful comment to that one:

/* system-wide limit on number of pages of shared memory; 0 means "no limit" */

Cheers,

Michael


>  #endif
>  #define SHMSEG SHMMNI                   /* max shared segs per process */
>
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 7645961..8630561 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -490,10 +490,12 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
>         int id;
>         vm_flags_t acctflag = 0;
>
> -       if (size < SHMMIN || size > ns->shm_ctlmax)
> +       if (size < SHMMIN ||
> +           (ns->shm_ctlmax && size > ns->shm_ctlmax))
>                 return -EINVAL;
>
> -       if (ns->shm_tot + numpages > ns->shm_ctlall)
> +       if (ns->shm_ctlall &&
> +           ns->shm_tot + numpages > ns->shm_ctlall)
>                 return -ENOSPC;
>
>         shp = ipc_rcu_alloc(sizeof(*shp));
> --
> 1.8.1.4
>
>
>
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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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