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    SubjectRe: [bigmem-patch] 4GB with Linux on IA32
    This other incremental patch will make the bigmem code safe w.r.t. raw-io:

    --- 2.3.13-bigmem-L/mm/memory.c Fri Aug 13 00:31:59 1999
    +++ 2.3.13-bigmem/mm/memory.c Tue Aug 17 00:59:37 1999
    @@ -436,6 +436,10 @@
    map = mem_map + MAP_NR(page);
    if (PageReserved(map))
    return 0;
    +#ifdef CONFIG_BIGMEM
    + if (PageBIGMEM(map))
    + return 0;
    +#endif
    return map;
    }

    But now IMO there's to choose between one of the below options:

    1) should we change all device drivers to allow us to do I/O over
    bigmem pages? NOTE: all DMA engine are just fine since virt_to_bus
    just works right as Gerhard pointed out to me. The only problem is for
    drivers that reads and writes to the b_data in software.
    2) should we change ll_rw_block to force an high limit of bh queued in
    the same request and then remap the b_data in the ll_rw_block layer
    with a NR_REQUEST*MAX_BH_PER_REQUEST array of virtual-pages in the
    fixmap area? (many tlb_flush_all... or at least many SMP-invlpg with a
    smarter cross-CPU-invlpg message)
    virt_to_bus must be able to resolve the bus address starting from
    the fixmap virtual address.
    3) using the remap trick that I am just using in the swapout/swapin code,
    I could just do raw-io on anonymous memory but I get stuck with the shm
    memory where I can't simply realloc a page without browsing all
    processes VM. Should I take a list of all pte that are mapping
    each smp page and doing the remap trick also on shm memory?
    4) should I avoid raw-io in the shm memory and use the remap trick
    with the anonymous memory?
    5) should I avoid bigmem in shm memory and simply use the remap trick
    with the anonymous memory?

    I guess big databases uses the shm memory as cache. And I guess they use
    raw-io to fill the shm memory with proper data. Am I right about this? If
    so I can't choose (4). And since I would like to use the bigmem as shm
    memory I would like to avoid also (5).

    (3) looks dirty and add a performance hit in the shm_nopage hander.

    (2) looks dirty and slow due the SMP tlb flushes.

    (1) looks clean and efficient (100% efficient in the DMA case!) but it
    breaks all drivers out there... :(((

    Theorically the cleanest solution would be (1) but I don't know if this
    will be a good choice on the long run (theorically on 2038 we won't need
    CONFIG_BIGMEM anymore...).

    Right now I temporary applyed solution (0): the patch at the top of this
    email so if you want to use raw-io on anonymous or shm memory you'll have
    to recompile with CONFIG_BIGMEM not set.

    Comments? (very welcome :)

    Andrea


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