• Mel Gorman's avatar
    [ARM] Double check memmap is actually valid with a memmap has unexpected holes V2 · eb33575c
    Mel Gorman authored
    pfn_valid() is meant to be able to tell if a given PFN has valid memmap
    associated with it or not. In FLATMEM, it is expected that holes always
    have valid memmap as long as there is valid PFNs either side of the hole.
    In SPARSEMEM, it is assumed that a valid section has a memmap for the
    entire section.
    However, ARM and maybe other embedded architectures in the future free
    memmap backing holes to save memory on the assumption the memmap is never
    used. The page_zone linkages are then broken even though pfn_valid()
    returns true. A walker of the full memmap must then do this additional
    check to ensure the memmap they are looking at is sane by making sure the
    zone and PFN linkages are still valid. This is expensive, but walkers of
    the full memmap are extremely rare.
    This was caught before for FLATMEM and hacked around but it hits again for
    SPARSEMEM because the page_zone linkages can look ok where the PFN linkages
    are totally screwed. This looks like a hatchet job but the reality is that
    any clean solution would end up consumning all the memory saved by punching
    these unexpected holes in the memmap. For example, we tried marking the
    memmap within the section invalid but the section size exceeds the size of
    the hole in most cases so pfn_valid() starts returning false where valid
    memmap exists. Shrinking the size of the section would increase memory
    consumption offsetting the gains.
    This patch identifies when an architecture is punching unexpected holes
    in the memmap that the memory model cannot automatically detect and sets
    ARCH_HAS_HOLES_MEMORYMODEL. At the moment, this is restricted to EP93xx
    which is the model sub-architecture this has been reported on but may expand
    later. When set, walkers of the full memmap must call memmap_valid_within()
    for each PFN and passing in what it expects the page and zone to be for
    that PFN. If it finds the linkages to be broken, it assumes the memmap is
    invalid for that PFN.
    Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
    Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
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