[RFC PATCH 2/3] topology: support node_numa_mem() for determining the fallback node

[David Rientjes]

On Thu, 6 Feb 2014, Joonsoo Kim wrote:

> From bf691e7eb07f966e3aed251eaeb18f229ee32d1f Mon Sep 17 00:00:00 2001
> From: Joonsoo Kim <iamjoonsoo.kim at lge.com>
> 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().