linux-kernel - Re: [PATCH 0/4] ipc/shm.c: increase the limits for SHMMAX, SHMALL

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Date:	Tue, 6 May 2014 22:40:23 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>, aswin@...com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 0/4] ipc/shm.c: increase the limits for SHMMAX, SHMALL

Hi Davidlohr,

On Tue, May 6, 2014 at 10:06 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> On Fri, 2014-05-02 at 15:16 +0200, Michael Kerrisk (man-pages) wrote:
>> Hi Manfred,
>>
>> On Mon, Apr 21, 2014 at 4:26 PM, Manfred Spraul
>> <manfred@...orfullife.com> wrote:
>> > Hi all,
>> >
>> > the increase of SHMMAX/SHMALL is now a 4 patch series.
>> > I don't have ideas how to improve it further.
>>
>> On the assumption that your patches are heading to mainline, could you
>> send me a man-pages patch for the changes?
>
> Btw, I think that the code could still use some love wrt documentation.

(Agreed.)

> Andrew, please consider this for -next if folks agree. Thanks.
>
> 8<----------------------------------------------------------
>
> From: Davidlohr Bueso <davidlohr@...com>
> Subject: [PATCH] ipc,shm: document new limits in the uapi header
>
> This is useful in the future and allows users to
> better understand the reasoning behind the changes.
>
> Also use UL as we're dealing with it anyways.
>
> Signed-off-by: Davidlohr Bueso <davidlohr@...com>
> ---
>  include/uapi/linux/shm.h | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
> index 74e786d..e37fb08 100644
> --- a/include/uapi/linux/shm.h
> +++ b/include/uapi/linux/shm.h
> @@ -8,17 +8,19 @@
>  #endif
>
>  /*
> - * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can

Something is wrong in the line above (missing word(s)?) ("are upper
limits are defaults")

> - * be modified by sysctl.
> + * SHMMNI, SHMMAX and SHMALL are the default upper limits which can be
> + * modified by sysctl. Both SHMMAX and SHMALL have their default values
> + * to the maximum limit which is as large as it can be without helping
> + * userspace overflow the values. There is really nothing the kernel
> + * can do to avoid this any variables. It is therefore not advised to

Something is missing in that last line.

> + * make them any larger. This is suitable for both 32 and 64-bit systems.

"This" is not so clear. I suggest replacing with an actual noun.

>   */
> -
>  #define SHMMIN 1                        /* min shared seg size (bytes) */
>  #define SHMMNI 4096                     /* max num of segs system wide */
> -#define SHMMAX (ULONG_MAX - (1L<<24))   /* max shared seg size (bytes) */
> -#define SHMALL (ULONG_MAX - (1L<<24))   /* max shm system wide (pages) */
> +#define SHMMAX (ULONG_MAX - (1UL << 24)) /* max shared seg size (bytes) */
> +#define SHMALL (ULONG_MAX - (1UL << 24)) /* max shm system wide (pages) */
>  #define SHMSEG SHMMNI                   /* max shared segs per process */
>
> -
>  /* Obsolete, used only for backwards compatibility and libc5 compiles */
>  struct shmid_ds {
>         struct ipc_perm         shm_perm;       /* operation perms */

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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