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

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Message-ID: <xr9361msjx94.fsf@gthelen.mtv.corp.google.com>
Date:	Tue, 01 Apr 2014 19:11:51 -0700
From:	Greg Thelen <gthelen@...gle.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Davidlohr Bueso <davidlohr@...com>,
	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\@kvack.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] ipc,shm: increase default size for shmmax


On Tue, Apr 01 2014, Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com> 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?
>> 
> (2014/04/02 10:08), Greg Thelen wrote:
>> 
>> 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.

Ah, yes. I forgot about shm_rmid_forced.  Thanks.  It would give the oom
killer ability to cleanup shm (as it does with anon) when
shm_rmid_forced=1.
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