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    SubjectRe: [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)
    On 04/02/20 at 10:01am, Michal Hocko wrote:
    > 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.

    Checked code again, seems HAVE_MEMBLOCK_NODE_MAP is selected directly in
    all ARCHes which support it. Means HAVE_MEMBLOCK_NODE_MAP is enabled by
    default on those ARCHes, and has no dependency on CONFIG_NUMA at all.
    E.g on x86, it just calls free_area_init_nodes() in generic code path,
    while free_area_init_nodes() is defined in CONFIG_HAVE_MEMBLOCK_NODE_MAP
    ifdeffery scope. So I tend to agree with Mike to remove
    HAVE_MEMBLOCK_NODE_MAP firstly on all ARCHes. We can check if it's worth
    only defining memblock_region->nid for CONFIG_NUMA case after
    HAVE_MEMBLOCK_NODE_MAP is removed.

    config X86
    def_bool y
    ...
    select HAVE_MEMBLOCK_NODE_MAP
    ...

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