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

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Date:	Thu, 11 Feb 2010 01:19:40 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Rik van Riel <riel@...hat.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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 Wed, 10 Feb 2010, Rik van Riel wrote:

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1914,6 +1914,9 @@ rebalance:
> >   	 * running out of options and have to consider going OOM
> >   	 */
> >   	if (!did_some_progress) {
> > +		/* The oom killer won't necessarily free lowmem */
> > +		if (high_zoneidx<  ZONE_NORMAL)
> > +			goto nopage;
> >   		if ((gfp_mask&  __GFP_FS)&&  !(gfp_mask&  __GFP_NORETRY)) {
> >   			if (oom_killer_disabled)
> >   				goto nopage;
> 
> Are there architectures that only have one memory zone?
> 

It actually ends up not to matter because of how gfp_zone() is implemented 
(and you can do it with mem= on architectures with larger ZONE_DMA zones 
such as ia64).  ZONE_NORMAL is always guaranteed to be defined regardless 
of architecture or configuration because it's the default zone for memory 
allocation unless specified by a bit in GFP_ZONEMASK, it doesn't matter 
whether it actually has memory or not.  high_zoneidx in this case is just 
gfp_zone(gfp_flags) which always defaults to ZONE_NORMAL when one of the 
GFP_ZONEMASK bits is not set.  Thus, the only way to for the conditional 
in this patch to be true is when __GFP_DMA, or __GFP_DMA32 for x86_64, is 
passed to the page allocator and CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is 
enabled, respectively.
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