linux-kernel - Re: OOM killer even when not overcommiting

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Date:	Mon, 5 Mar 2012 22:13:11 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: OOM killer even when not overcommiting

On Mon, Mar 5, 2012 at 10:04 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Mon, 05 Mar 2012 21:58:26 +0200
> Sasha Levin <levinsasha928@...il.com> wrote:
>
>> Hi all,
>
>> I assumed that when setting overcommit_memory=2 and
>> overcommit_ratio<100 that the OOM killer won't ever get invoked (since
>> we're not overcommiting memory), but it looks like I'm mistaken since
>> apparently a simple mmap from userspace will trigger the OOM killer if
>> it requests more memory than available.
>>
>> Is it how it's supposed to work?  Why does it resort to OOM killing
>> instead of just failing the allocation?
>>
>> Here is the dump I get when the OOM kicks in:
>>
>> ...
>>
>> [ 3108.730350]  [<ffffffff81198e4a>] mlock_vma_pages_range+0x9a/0xa0
>> [ 3108.734486]  [<ffffffff8119b75b>] mmap_region+0x28b/0x510
>> ...
>
> The vma is mlocked for some reason - presumably the app is using
> mlockall() or mlock()?  So the kernel is trying to instantiate all the
> pages at mmap() time.

The app may have used mlock(), but there is no swap space on the
machine (it's also a KVM guest), so it should matter, no?

Regardless, why doesn't it result in mmap() failing quietly, instead
of kicking in the OOM killer to kill the entire process?
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