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    SubjectRe : sparsemem usage
    KAMEZAWA Hiroyuki wrote:
    > On Thu, 10 Aug 2006 14:40:52 +0200 (CEST)
    > moreau francis <francis_moreau2000@yahoo.fr> wrote:
    >>> BTW, ioresouce information (see kernel/resouce.c)
    >>>
    >>> [kamezawa@aworks Development]$ cat /proc/iomem | grep RAM
    >>> 00000000-0009fbff : System RAM
    >>> 000a0000-000bffff : Video RAM area
    >>> 00100000-2dfeffff : System RAM
    >>>
    >>> is not enough ?
    >>>
    >> well actually you show that to get a really simple information, ie does
    >> a page exist ?, we need to parse some kernel data structures like
    >> ioresource (which is, IMHO, hackish) or duplicate in each architecture
    >> some data to keep track of existing pages.
    >>
    >
    > becasue memory map from e820(x86) or efi(ia64) are registered to iomem_resource,
    > we should avoid duplicates that information. kdump and memory hotplug uses
    > this information. (memory hotplug updates this iomem_resource.)
    >
    > Implementing "page_is_exist" function based on ioresouce is one of generic
    > and rubust way to go, I think.
    > (if performance of list walking is problem, enhancing ioresouce code is
    > better.)
    >

    Why not implementing page_exist() by simply using mem_map[] ? When
    allocating mem_map[], we can just fill it with a special value. And
    then when registering memory area, we clear this special value with
    the "reserved" value. Hence for flatmem model, we can have:

    #define page_exist(pfn) (mem_map[pfn] != SPECIAL_VALUE)

    and it should work for sparsemem too and other models that will use
    mem_map[].

    Another point, is page_exist() going to replace page_valid() ?
    I mean page_exist() is going to be something more accurate than
    page_valid(). All tests on page_valid() _only_ will be fine to test
    page_exist(). But all tests such:

    if (page_valid(x) && page_is_ram(x))

    can be replaced by

    if (page_exist(x))

    So, again, why not simply improving page_valid() definition rather
    than introduce a new service ?

    Francis




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