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    SubjectRe: [PATCH 1/2] Align the node_mem_map endpoints to a MAX_ORDER boundary
    Andrew Morton wrote:
    > Mel Gorman <mel@csn.ul.ie> wrote:
    >
    >>Andy added code to buddy allocator which does not require the zone's
    >>endpoints to be aligned to MAX_ORDER. An issue is that the buddy
    >>allocator requires the node_mem_map's endpoints to be MAX_ORDER aligned.
    >>Otherwise __page_find_buddy could compute a buddy not in node_mem_map for
    >>partial MAX_ORDER regions at zone's endpoints. page_is_buddy will detect
    >>that these pages at endpoints are not PG_buddy (they were zeroed out by
    >>bootmem allocator and not part of zone). Of course the negative here is
    >>we could waste a little memory but the positive is eliminating all the
    >>old checks for zone boundary conditions.
    >>
    >>SPARSEMEM won't encounter this issue because of MAX_ORDER size constraint
    >>when SPARSEMEM is configured. ia64 VIRTUAL_MEM_MAP doesn't need the
    >>logic either because the holes and endpoints are handled differently.
    >>This leaves checking alloc_remap and other arches which privately allocate
    >>for node_mem_map.
    >
    >
    > Do we think we need this in 2.6.17?

    I would say yes, it is a very low risk patch in my view and provides a
    very large part of the protections we require. i386 as our largest
    userbase should be safe from zone/node alignment issues with just this
    change. Others need slightly more (the page_zone_idx check) which is
    being discussed in another thread.

    -apw
    -
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