Re: [PATCH RFC] mm: remove CONFIG_HAVE_MEMBLOCK_NODE_MAP (was: Re: [PATCH v3 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by default for NUMA)

[Michal Hocko]

On Wed 01-04-20 10:51:55, Mike Rapoport wrote:
> Hi,
> 
> On Wed, Apr 01, 2020 at 01:42:27PM +0800, Baoquan He wrote:
[...]
> > From above information, we can remove HAVE_MEMBLOCK_NODE_MAP, and
> > replace it with CONFIG_NUMA. That sounds more sensible to store nid into
> > memblock when NUMA support is enabled.
>  
> Replacing CONFIG_HAVE_MEMBLOCK_NODE_MAP with CONFIG_NUMA will work, but
> this will not help cleaning up the whole node/zone initialization mess and
> we'll be stuck with two implementations.

Yeah, this is far from optimal.

> The overhead of enabling HAVE_MEMBLOCK_NODE_MAP is only for init time as
> most architectures will anyway discard the entire memblock, so having it in
> a UMA arch won't be a problem. The only exception is arm that uses
> memblock for pfn_valid(), here we may also think about a solution to
> compensate the addition of nid to the memblock structures. 

Well, we can make memblock_region->nid defined only for CONFIG_NUMA.
memblock_get_region_node would then unconditionally return 0 on UMA.
Essentially the same way we do NUMA for other MM code. I only see few
direct usage of region->nid.
-- 
Michal Hocko
SUSE Labs