linux-kernel - Re: [RFC PATCH] mm: exclude memory less nodes from zone

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Message-ID: <alpine.DEB.2.02.1402191353540.31921@chino.kir.corp.google.com>
Date:	Wed, 19 Feb 2014 13:56:00 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
cc:	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: exclude memory less nodes from zone_reclaim

On Wed, 19 Feb 2014, Nishanth Aravamudan wrote:

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3e953f07edb0..4a44bdc7a8cf 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1855,7 +1855,7 @@ static void __paginginit init_zone_allows_reclaim(int nid)
> >  {
> >  	int i;
> > 
> > -	for_each_online_node(i)
> > +	for_each_node_state(i, N_HIGH_MEMORY)
> >  		if (node_distance(nid, i) <= RECLAIM_DISTANCE)
> >  			node_set(i, NODE_DATA(nid)->reclaim_nodes);
> >  		else
> > @@ -4901,7 +4901,8 @@ void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
> > 
> >  	pgdat->node_id = nid;
> >  	pgdat->node_start_pfn = node_start_pfn;
> > -	init_zone_allows_reclaim(nid);
> > +	if (node_state(nid, N_HIGH_MEMORY))
> > +		init_zone_allows_reclaim(nid);
> 
> I'm still new to this code, but isn't this saying that if a node has no
> memory, then it shouldn't reclaim from any node? But, for a memoryless
> node to ensure progress later if reclaim is necessary, it *must* reclaim
> from other nodes? So wouldn't we want to set reclaim_nodes() in that
> case to node_states[N_MEMORY]?
> 

The only time when pgdat->reclaim_nodes or zone_reclaim_mode matters is 
when iterating through a zonelist for page allocation and a memoryless 
node should never appear in a zonelist for page allocation, so this is 
just preventing setting zone_reclaim_mode unnecessarily because the only 
nodes with > RECLAIM_DISTANCE to another node are memoryless.  So this 
patch is fine as long as it gets s/N_HIGH_MEMORY/N_MEMORY/.

Acked-by: David Rientjes <rientjes@...gle.com>
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