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    SubjectRe: [PATCH 1/5] x86: Change size of node ids from u8 to u16 fixup
    On Sat, 19 Jan 2008, Mike Travis wrote:

    > > Yeah, NID_INVAL is negative so no unsigned type will work here,
    > > unfortunately. And that reduces the intended savings of your change since
    > > the smaller type can only be used with a smaller CONFIG_NODES_SHIFT.
    > >
    >
    > Excuse my ignorance but why wouldn't this work:
    >
    > static numanode_t pxm_to_node_map[MAX_PXM_DOMAINS]
    > = { [0 ... MAX_PXM_DOMAINS - 1] = NUMA_NO_NODE };
    > ...
    > >> int acpi_map_pxm_to_node(int pxm)
    > >> {
    > > int node = pxm_to_node_map[pxm];
    > >
    > > if (node < 0)
    >
    > numanode_t node = pxm_to_node_map[pxm];
    >

    Because NUMA_NO_NODE is 0xff on x86. That's a valid node id for
    configurations with CONFIG_NODES_SHIFT equal to or greater than 8.

    > if (node != NUMA_NO_NODE) {

    Wrong, this should be

    node == NUMA_NO_NODE

    > >> if (nodes_weight(nodes_found_map) >= MAX_NUMNODES)
    > >> return NID_INVAL;
    > >> node = first_unset_node(nodes_found_map);
    > >> __acpi_map_pxm_to_node(pxm, node);
    > >> node_set(node, nodes_found_map);
    > >> }
    >

    The net result of this is that if a proximity domain is looked up through
    acpi_map_pxm_to_node() and already has a mapping to node 255 (legal with
    CONFIG_NODES_SHIFT == 8), this function will return NID_INVAL since the
    weight of nodes_found_map is equal to MAX_NUMNODES.

    You simply can't use valid node id's to signify invalid or unused node
    ids.

    > or change:
    > #define NID_INVAL (-1)
    > to
    > #define NID_INVAL ((numanode_t)(-1))
    > ...
    > if (node != NID_INVAL) {

    You mean

    node == NID_INVAL

    > >> if (nodes_weight(nodes_found_map) >= MAX_NUMNODES)
    > >> return NID_INVAL;
    > >> node = first_unset_node(nodes_found_map);
    > >> __acpi_map_pxm_to_node(pxm, node);
    > >> node_set(node, nodes_found_map);
    > >> }
    >

    That's the equivalent of your NUMA_NO_NODE code above. The fact remains
    that (numanode_t)-1 is still a valid node id for MAX_NUMNODES >= 256.

    So, as I said in my initial reply, the only way to get the savings you're
    looking for is to use u8 for CONFIG_NODES_SHIFT <= 7 and then convert all
    NID_INVAL users to use NUMA_NO_NODE.

    Additionally, Linux has always discouraged typedefs when they do not
    define an architecture-specific size. The savings from your patch for
    CONFIG_NODES_SHIFT == 7 would be 256 bytes for this mapping.

    It's simply not worth it.

    > And btw, shouldn't the pxm value be sized to numanode_t size as well?
    > Will it ever be larger than the largest node id?
    >

    Section 6.2.9 of ACPI 2.0 states that PXM's return an integer, so that
    would be non-conforming to the standard.

    Additionally, PXM's are not nodes, so casting them to anything called
    numanode_t shows the semantic flaw in your patch.

    David


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