On Thu, 6 Feb 2014, Joonsoo Kim wrote:

From bf691e7eb07f966e3aed251eaeb18f229ee32d1f Mon Sep 17 00:00:00 2001
From: Joonsoo Kim <redacted>
Date: Thu, 6 Feb 2014 17:07:05 +0900
Subject: [RFC PATCH 2/3 v2] topology: support node_numa_mem() for
determining the
 fallback node

We need to determine the fallback node in slub allocator if the allocation
target node is memoryless node. Without it, the SLUB wrongly select
the node which has no memory and can't use a partial slab, because of node
mismatch. Introduced function, node_numa_mem(X), will return
a node Y with memory that has the nearest distance. If X is memoryless
node, it will return nearest distance node, but, if
X is normal node, it will return itself.

We will use this function in following patch to determine the fallback
node.

I like the approach and it may fix the problem today, but it may not be 
sufficient in the future: nodes may not only be memoryless but they may 
also be cpuless.  It's possible that a node can only have I/O, networking, 
or storage devices and we can define affinity for them that is remote from 
every cpu and/or memory by the ACPI specification.

It seems like a better approach would be to do this when a node is brought 
online and determine the fallback node based not on the zonelists as you 
do here but rather on locality (such as through a SLIT if provided, see 
node_distance()).

Also, the names aren't very descriptive: {get,set}_numa_mem() doesn't make 
a lot of sense in generic code.  I'd suggest something like 
node_to_mem_node().