linux-kernel - Re: [patch 6/7 -mm] oom: avoid oom killer for lowmem allocations

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Message-ID: <alpine.DEB.2.00.1002120200050.22883@chino.kir.corp.google.com>
Date:	Fri, 12 Feb 2010 02:06:49 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Lubos Lunak <l.lunak@...e.cz>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch 6/7 -mm] oom: avoid oom killer for lowmem allocations

On Fri, 12 Feb 2010, KAMEZAWA Hiroyuki wrote:

> From viewpoint of panic-on-oom lover, this patch seems to cause regression.
> please do this check after sysctl_panic_on_oom == 2 test.
> I think it's easy. So, temporary Nack to this patch itself.
> 
> 
> And I think calling notifier is not very bad in the situation.
> ==
> void out_of_memory()
>  ..snip..
>   blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
> 
> 
> So,
> 
>         if (sysctl_panic_on_oom == 2) {
>                 dump_header(NULL, gfp_mask, order, NULL);
>                 panic("out of memory. Compulsory panic_on_oom is selected.\n");
>         }
> 
> 	if (gfp_zone(gfp_mask) < ZONE_NORMAL) /* oom-kill is useless if lowmem is exhausted. */
> 		return;
> 
> is better. I think.
> 

I can't agree with that assessment, I don't think it's a desired result to 
ever panic the machine regardless of what /proc/sys/vm/panic_on_oom is set 
to because a lowmem page allocation fails especially considering, as 
mentioned in the changelog, these allocations are never __GFP_NOFAIL and 
returning NULL is acceptable.

I've always disliked panicking the machine when a cpuset or mempolicy 
allocation fails and panic_on_oom is set to 2.  Since both such 
constraints now force an iteration of the tasklist when oom_kill_quick is 
not enabled and we strictly prohibit the consideration of tasks with 
disjoint cpuset mems or mempolicy nodes, I think I'll take this 
opportunity to get rid of the panic_on_oom == 2 behavior and ask that 
users who really do want to panic the entire machine for cpuset or 
mempolicy constrained ooms to simply set all such tasks to OOM_DISABLE.
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