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

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Message-Id: <20120305120427.2d11d30e.akpm@linux-foundation.org>
Date:	Mon, 5 Mar 2012 12:04:27 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sasha Levin <levinsasha928@...il.com>
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, 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.


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