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    SubjectRe: [PATCH] x86: Construct 32 bit boot time page tables in native format.

    On Sun, 2008-01-20 at 20:30 +0200, Mika Penttilä wrote:
    > > + * This is how much memory *in addition to the memory covered up to
    > > + * and including _end* we need mapped initially. We need one bit for
    > > + * each possible page, but only in low memory, which means
    > > + * 2^32/4096/8 = 128K worst case (4G/4G split.)
    > > + *
    > > + * Modulo rounding, each megabyte assigned here requires a kilobyte of
    > > + * memory, which is currently unreclaimed.
    > > + *
    > > + * This should be a multiple of a page.
    > > + */
    > > +#define INIT_MAP_BEYOND_END (128*1024)
    > > +
    > > +/*
    > >
    > You have dropped the requirement to map all of low memory (the boot
    > allocator is used for instance to construct physical mem mapping).
    > Either you should fix your INIT_MAP_BEYOND_END or make a big comment
    > telling us why it isn't necessary anymore to map low mem.

    I think you are right. The patch ensures that all the initial page
    tables themselves have mappings but won't map the additional pages
    needed for mapping the rest of lowmem.

    However, I think it is no longer necessary to map a whole new 4G worth
    of page table pages because the code in kernel_physical_mapping_init now
    extends the initial mappings rather than replacing them (see changes to
    native_pagetable_setup_start). So now we only need to map 4G worth of
    page tables including the initial page tables. That means we only need
    to map a fixed set of extra pages rather than the sliding limit
    currently used in the patch.

    I'm not convinced by the additional 16MB for CONFIG_DEBUG_PAGEALLOC --
    we map enough pages for page tables for 4G of lowmem -- adding space for
    an extra 16M seems pointless.

    Ian Campbell

    Good-bye. I am leaving because I am bored.
    -- George Saunders' dying words

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