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

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Date:	Fri, 13 May 2016 10:14:55 -0400
From:	"Austin S. Hemmelgarn" <ahferroin7@...il.com>
To:	Sebastian Frias <sf84@...oste.net>,
	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

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.

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.  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.