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    Subject[PATCH 2/2] FLATMEM relax requirement for memory to start at pfn 0
    Date

    From: Andy Whitcroft <apw@shadowen.org>

    The FLATMEM memory model assumes that memory is in one contigious area
    based at pfn 0. If we initialise node 0 to start at any other offset we
    will incorrectly map pfn's to the wrong struct page *. The key to the
    memory model is the contigious nature of the memory not the location of it.
    Relax the requirement for the area to start at 0.


    page_alloc.c | 17 +++++++++++++----
    1 files changed, 13 insertions(+), 4 deletions(-)

    Signed-off-by: Andy Whitcroft <apw@shadowen.org>
    Acked-by: Mel Gorman <mel@csn.ul.ie>

    diff -rup -X /usr/src/patchset-0.5/bin//dontdiff linux-2.6.17-rc4-mm1-101-bob-node-alignment/mm/page_alloc.c linux-2.6.17-rc4-mm1-102-FLATMEM-relax-requirement-for-memory-to-start-at-pfn-0/mm/page_alloc.c
    --- linux-2.6.17-rc4-mm1-101-bob-node-alignment/mm/page_alloc.c 2006-05-18 17:58:10.000000000 +0100
    +++ linux-2.6.17-rc4-mm1-102-FLATMEM-relax-requirement-for-memory-to-start-at-pfn-0/mm/page_alloc.c 2006-05-18 19:14:44.000000000 +0100
    @@ -2477,15 +2477,16 @@ static void __meminit free_area_init_cor

    static void __init alloc_node_mem_map(struct pglist_data *pgdat)
    {
    +#ifdef CONFIG_FLAT_NODE_MEM_MAP
    + struct page *map = pgdat->node_mem_map;
    +
    /* Skip empty nodes */
    if (!pgdat->node_spanned_pages)
    return;

    -#ifdef CONFIG_FLAT_NODE_MEM_MAP
    /* ia64 gets its own node_mem_map, before this, without bootmem */
    - if (!pgdat->node_mem_map) {
    + if (!map) {
    unsigned long size, start, end;
    - struct page *map;

    /*
    * The zone's endpoints aren't required to be MAX_ORDER
    @@ -2500,13 +2501,21 @@ static void __init alloc_node_mem_map(st
    if (!map)
    map = alloc_bootmem_node(pgdat, size);
    pgdat->node_mem_map = map + (pgdat->node_start_pfn - start);
    +
    + /*
    + * With FLATMEM the global mem_map is used. This is assumed
    + * to be based at pfn 0 such that 'pfn = page* - mem_map'
    + * is true. Adjust map relative to node_mem_map to
    + * maintain this relationship.
    + */
    + map -= pgdat->node_start_pfn;
    }
    #ifdef CONFIG_FLATMEM
    /*
    * With no DISCONTIG, the global mem_map is just set as node 0's
    */
    if (pgdat == NODE_DATA(0))
    - mem_map = NODE_DATA(0)->node_mem_map;
    + mem_map = map;
    #endif
    #endif /* CONFIG_FLAT_NODE_MEM_MAP */
    }
    -
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