On Sat, 28 May 2011, Michal Hocko wrote:
> > CONFIG_NODES_SHIFT is used for UMA machines that are using DISCONTIGMEM
> > usually because they have very large holes; such machines don't need
> > things like mempolicies but do need the data structures that abstract
> > ranges of memory in the physical address space. This build breakage
> > probably isn't restricted to only alpha, you could probably see it with at
> > least ia64 and mips as well.
>
> Hmmm. I just find strange that some UMA arch uses functions like
> {first,next}_online_node.
>
They shouldn't, but they do use NUMA data structures like pg_data_t for
DISCONTIGMEM. The MAX_NUMNODES > 1 optimization in nodemask.h is to
prevent doing things like node_weight() on a nodemask when we know that
only one bit will ever be set, otherwise we could make it conditional on
CONFIG_NEED_MULTIPLE_NODES.
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