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    Subject[patch] x86: allow ZONE_DMA to be configurable
    ZONE_DMA is unnecessary for a large number of machines that do not
    require addressing in the lower 16MB of memory because they do not use
    ISA devices with 16-bit address registers (plus one page byte register).

    This patch allows users to disable ZONE_DMA for x86 if they know they
    will not be using such devices with their kernel.

    This prevents the VM from unnecessarily reserving a ratio of memory
    (defaulting to 1/256th of system capacity) with lowmem_reserve_ratio
    for such allocations when it will never be used.

    Signed-off-by: David Rientjes <rientjes@google.com>
    ---
    arch/x86/Kconfig | 7 ++++++-
    arch/x86/mm/init_32.c | 2 ++
    arch/x86/mm/init_64.c | 2 ++
    3 files changed, 10 insertions(+), 1 deletions(-)

    diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -99,7 +99,12 @@ config MMU
    def_bool y

    config ZONE_DMA
    - def_bool y
    + bool "DMA memory allocation support"
    + default y
    + help
    + DMA memory allocation support allows old ISA devices with 16-bit
    + address registers to allocate memory within the first 16MB of
    + address space. Disable if no such devices will be used.

    config SBUS
    bool
    diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
    --- a/arch/x86/mm/init_32.c
    +++ b/arch/x86/mm/init_32.c
    @@ -740,8 +740,10 @@ static void __init zone_sizes_init(void)
    {
    unsigned long max_zone_pfns[MAX_NR_ZONES];
    memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
    +#ifdef CONFIG_ZONE_DMA
    max_zone_pfns[ZONE_DMA] =
    virt_to_phys((char *)MAX_DMA_ADDRESS) >> PAGE_SHIFT;
    +#endif
    max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
    #ifdef CONFIG_HIGHMEM
    max_zone_pfns[ZONE_HIGHMEM] = highend_pfn;
    diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
    --- a/arch/x86/mm/init_64.c
    +++ b/arch/x86/mm/init_64.c
    @@ -598,7 +598,9 @@ void __init paging_init(void)
    unsigned long max_zone_pfns[MAX_NR_ZONES];

    memset(max_zone_pfns, 0, sizeof(max_zone_pfns));
    +#ifdef CONFIG_ZONE_DMA
    max_zone_pfns[ZONE_DMA] = MAX_DMA_PFN;
    +#endif
    max_zone_pfns[ZONE_DMA32] = MAX_DMA32_PFN;
    max_zone_pfns[ZONE_NORMAL] = max_pfn;


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