linux-kernel - Re: [PATCH] ipc,shm: increase default size for shmmax

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Date:	Wed, 02 Apr 2014 10:58:24 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Greg Thelen <gthelen@...gle.com>,
	Davidlohr Bueso <davidlohr@...com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>, aswin@...com,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax

(2014/04/02 10:08), Greg Thelen wrote:
> 
> On Tue, Apr 01 2014, Davidlohr Bueso <davidlohr@...com> wrote:
> 
>> On Tue, 2014-04-01 at 19:56 -0400, KOSAKI Motohiro wrote:
>>>>>> Ah-hah, that's interesting info.
>>>>>>
>>>>>> Let's make the default 64GB?
>>>>>
>>>>> 64GB is infinity at that time, but it no longer near infinity today. I like
>>>>> very large or total memory proportional number.
>>>>
>>>> So I still like 0 for unlimited. Nice, clean and much easier to look at
>>>> than ULONG_MAX. And since we cannot disable shm through SHMMIN, I really
>>>> don't see any disadvantages, as opposed to some other arbitrary value.
>>>> Furthermore it wouldn't break userspace: any existing sysctl would
>>>> continue to work, and if not set, the user never has to worry about this
>>>> tunable again.
>>>>
>>>> Please let me know if you all agree with this...
>>>
>>> Surething. Why not. :)
>>
>> *sigh* actually, the plot thickens a bit with SHMALL (total size of shm
>> segments system wide, in pages). Currently by default:
>>
>> #define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16))
>>
>> This deals with physical memory, at least admins are recommended to set
>> it to some large percentage of ram / pagesize. So I think that if we
>> loose control over the default value, users can potentially DoS the
>> system, or at least cause excessive swapping if not manually set, but
>> then again the same goes for anon mem... so do we care?
> 
> At least when there's an egregious anon leak the oom killer has the
> power to free the memory by killing until the memory is unreferenced.
> This isn't true for shm or tmpfs.  So shm is more effective than anon at
> crushing a machine.

Hm..sysctl.kernel.shm_rmid_forced won't work with oom-killer ?

http://www.openwall.com/lists/kernel-hardening/2011/07/26/7

I like to handle this kind of issue under memcg but hmm..tmpfs's limit is half
of memory at default.

Thanks,
-Kame








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