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    Subject[PATCH 0/3] Zone boundry alignment fixes
    Ok.  Finally got my test bed working and got this lot tested.

    To summarise the problem , the buddy allocator currently requires
    that the boundries between zones occur at MAX_ORDER boundries.
    The specific case where we were tripping up on this was in x86 with
    NUMA enabled. There we try to ensure that each node's stuct pages
    are in node local memory, in order to allow them to be virtually
    mapped we have to reduce the size of ZONE_NORMAL. Here we are
    rounding the remap space up to a large page size to allow large
    page TLB entries to be used. However, these are smaller than
    MAX_ORDER. This can lead to bad buddy merges. With VM_DEBUG enabled
    we detect the attempts to merge across this boundry and panic.

    We have two basic options we can either apply the appropriate
    alignment when we make make the NUMA remap space, or we can 'fix'
    the assumption in the buddy allocator. The fix for the buddy
    allocator involves adding conditionals to the free fast path and
    so it seems reasonable to at least favor realigning the remap space.

    Following this email are 3 patches:

    zone-init-check-and-report-unaligned-zone-boundries -- introduces
    a zone alignement helper, and uses it to add a check to zone
    initialisation for unaligned zone boundries,

    x86-align-highmem-zone-boundries-with-NUMA -- uses the zone alignment
    helper to align the end of ZONE_NORMAL after the remap space has
    been reserved, and

    zone-allow-unaligned-zone-boundries -- modifies the buddy allocator
    so that we can allow unaligned zone boundries. A new configuration
    option is added to enable this functionality.

    The first two are the fixes for alignement in x86, these fix the
    panics thrown when VM_DEBUG is enabled.

    The last is a patch to support unaligned zone boundries. As this
    (re)introduces a zone check into the free hot path it seems
    reasonable to only enable this should it be needed; for example
    we never need this if we have a single zone. I have tested the
    failing system with this patch enabled and it also fixes the panic.
    I am inclined to suggest that it be included as it very clearly
    documents the alignment requirements for the buddy allocator.


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      Last update: 2006-05-09 13:11    [W:0.032 / U:31.904 seconds]
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