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

[Kamezawa Hiroyuki]

(2014/04/03 9:20), Davidlohr Bueso wrote:
> 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
> kill processes that are hogging memory through shm, allowing users to
> potentially abuse this. To overcome this situation, the shm_rmid_forced
> option must be enabled.
>
> Running this patch through LTP, everything passes, except the following,
> which, due to the nature of this change, is quite expected:
>
> shmget02    1  TFAIL  :  call succeeded unexpectedly
>
> [1]: http://rhaas.blogspot.com/2012/06/absurd-shared-memory-limits.html
>
> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>

looks good to me
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

When this goes mainline, updating a man page of ipcs may be required.

Thanks,
-Kame

> ---
>   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
> + * 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) */
>   #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
>   #endif
>   #define SHMSEG SHMMNI			 /* max shared segs per process */
>
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 7645961..ae01ffa 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 (ns->shm_ctlmax &&
> +	    (size < SHMMIN || 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));


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