linux-kernel - Re: [PATCH] mm: add config option to select the initial overcommit mode

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Date:	Fri, 13 May 2016 16:23:46 +0200
From:	Sebastian Frias <sf84@...oste.net>
To:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>,
	Michal Hocko <mhocko@...nel.org>
CC:	Mason <slash.tmp@...e.fr>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: add config option to select the initial overcommit
 mode

Hi Austin,

On 05/13/2016 04:14 PM, Austin S. Hemmelgarn wrote:
> On 2016-05-13 09:34, Sebastian Frias wrote:
>> Hi Austin,
>>
>> On 05/13/2016 03:11 PM, Austin S. Hemmelgarn wrote:
>>> On 2016-05-13 08:39, Sebastian Frias wrote:
>>>>
>>>> My point is that it seems to be possible to deal with such conditions in a more controlled way, ie: a way that is less random and less abrupt.
>>> There's an option for the OOM-killer to just kill the allocating task instead of using the scoring heuristic.  This is about as deterministic as things can get though.
>>
>> By the way, why does it has to "kill" anything in that case?
>> I mean, shouldn't it just tell the allocating task that there's not enough memory by letting malloc return NULL?
> In theory, that's a great idea.  In practice though, it only works if:
> 1. The allocating task correctly handles malloc() (or whatever other function it uses) returning NULL, which a number of programs don't.
> 2. The task actually has fallback options for memory limits.  Many programs that do handle getting a NULL pointer from malloc() handle it by exiting anyway, so there's not as much value in this case.
> 3. There isn't a memory leak somewhere on the system.  Killing the allocating task doesn't help much if this is the case of course.

Well, the thing is that the current behaviour, i.e.: overcommiting, does not improves the quality of those programs.
I mean, what incentive do they have to properly handle situations 1, 2?

Also, if there's a memory leak, the termination of any task, whether it is the allocating task or something random, does not help either, the system will eventually go down, right?

> 
> You have to keep in mind though, that on a properly provisioned system, the only situations where the OOM killer should be invoked are when there's a memory leak, or when someone is intentionally trying to DoS the system through memory exhaustion. 

Exactly, the DoS attack is another reason why the OOM-killer does not seem a good idea, at least compared to just letting malloc return NULL and let the program fail.

>If you're hitting the OOM killer for any other reason than those or a kernel bug, then you just need more memory or more swap space.
> 

Indeed.

Best regards,

Sebastian