Re: [RFC PATCH] powerpc/numa: reset node_possible_map to only node_online_map

On Fri, 6 Mar 2015, Michael Ellerman wrote:

quoted

quoted

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 0257a7d659ef..24de29b3651b 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c

@@ -958,9 +958,17 @@ void __init initmem_init(void)
 
 	memblock_dump_all();
 
+	/*
+	 * zero out the possible nodes after we parse the device-tree,
+	 * so that we lower the maximum NUMA node ID to what is actually
+	 * present.
+	 */
+	nodes_clear(node_possible_map);
+
 	for_each_online_node(nid) {
 		unsigned long start_pfn, end_pfn;
 
+		node_set(nid, node_possible_map);
 		get_pfn_range_for_nid(nid, &start_pfn, &end_pfn);
 		setup_node_data(nid, start_pfn, end_pfn);
 		sparse_memory_present_with_active_regions(nid);

This seems a bit strange, node_possible_map is supposed to be a superset 
of node_online_map and this loop is iterating over node_online_map to set 
nodes in node_possible_map.

 
Yeah. Though at this point in boot I don't think it matters that the two maps
are out-of-sync temporarily.

But it would simpler to just set the possible map to be the online map. That
would also maintain the invariant that the possible map is always a superset of
the online map.

Or did I miss a detail there (sleep deprived parent mode).

I think reset_numa_cpu_lookup_table() which iterates over the possible 
map, and thus only a subset of nodes now, may be concerning.

I'm not sure why this is being proposed as a powerpc patch and now a patch 
for mem_cgroup_css_alloc().  In other words, why do we have to allocate 
for all possible nodes?  We should only be allocating for online nodes in 
N_MEMORY with mem hotplug disabled initially and then have a mem hotplug 
callback implemented to alloc_mem_cgroup_per_zone_info() for nodes that 
transition from memoryless -> memory.  The extra bonus is that 
alloc_mem_cgroup_per_zone_info() need never allocate remote memory and the 
TODO in that function can be removed.