Re: [RFC PATCH] mm: exclude memory less nodes from zone_reclaim

Nishanth Aravamudan
2014-02-19T23:06:09Z

On 19.02.2014 [13:56:00 -0800], David Rientjes wrote:
> 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/.

Ah yes, sorry, I've been looking at this code perhaps too much and going
a bit cross-eyed!

I wonder if we should also put some comments in? But

Acked-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Tested-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>

Thanks,
Nish