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

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Date:	Thu, 3 Apr 2014 19:39:38 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aswin@...com,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Greg Thelen <gthelen@...gle.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] ipc,shm: disable shmmax and shmall by default

On Thu, Apr 3, 2014 at 3:50 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> On Thu, 2014-04-03 at 21:02 +0200, Manfred Spraul wrote:
>> Hi Davidlohr,
>>
>> On 04/03/2014 02:20 AM, 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.
>
>> Are we sure that no user space apps uses shmctl(IPC_INFO) and prints a
>> pretty error message if shmall is too small?
>> We would break these apps.
>
> Good point. 0 bytes/pages would definitely trigger an unexpected error
> message if users did this. But on the other hand I'm not sure this
> actually is a _real_ scenario, since upon overflow the value can still
> end up being 0, which is totally bogus and would cause the same
> breakage.
>
> So I see two possible workarounds:
> (i) Use ULONG_MAX for the shmmax default instead. This would make shmall
> default to 1152921504606846720 and 268435456, for 64 and 32bit systems,
> respectively.
>
> (ii) Keep the 0 bytes, but add a new a "transition" tunable that, if set
> (default off), would allow 0 bytes to be unlimited. With time, users
> could hopefully update their applications and we could eventually get
> rid of it. This _seems_ to be the less aggressive way to go.

Do you mean

set 0: IPC_INFO return shmmax = 0.
set 1: IPC_INFO return shmmax = ULONG_MAX.

?

That makes sense.
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