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    SubjectRE: [PATCH] arm64: mm: decrease the section size to reduce the memory reserved for the page map
    Date


    > -----Original Message-----
    > From: liwei (CM)
    > Sent: Friday, December 4, 2020 2:45 PM
    > To: catalin.marinas@arm.com; rppt@linux.ibm.com; will@kernel.org; liwei (CM)
    > <liwei213@huawei.com>
    > Cc: fengbaopeng <fengbaopeng2@hisilicon.com>; nsaenzjulienne@suse.de;
    > steve.capper@arm.com; Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>;
    > linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; butao
    > <butao@hisilicon.com>
    > Subject: [PATCH] arm64: mm: decrease the section size to reduce the memory
    > reserved for the page map
    >
    > For the memory hole, sparse memory model that define SPARSEMEM_VMEMMAP
    > do not free the reserved memory for the page map, decrease the section
    > size can reduce the waste of reserved memory.
    >
    > Signed-off-by: Wei Li <liwei213@huawei.com>
    > Signed-off-by: Baopeng Feng <fengbaopeng2@hisilicon.com>
    > Signed-off-by: Xia Qing <saberlily.xia@hisilicon.com>
    > ---

    Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>

    When page size is 4K, for each 1GB memory, we need 16MB vmemmap;
    For each 128MB memory, we need 2MB vmemmap.

    So while we have memory hole like 928MB(1GB-64MB),if SECTION_SIZE_BITS
    is 30, we waste 15MB; if SECTION_SIZE_BITS is 27, we waste 1MB only.

    > arch/arm64/include/asm/sparsemem.h | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/arch/arm64/include/asm/sparsemem.h
    > b/arch/arm64/include/asm/sparsemem.h
    > index 1f43fcc79738..8963bd3def28 100644
    > --- a/arch/arm64/include/asm/sparsemem.h
    > +++ b/arch/arm64/include/asm/sparsemem.h
    > @@ -7,7 +7,7 @@
    >
    > #ifdef CONFIG_SPARSEMEM
    > #define MAX_PHYSMEM_BITS CONFIG_ARM64_PA_BITS
    > -#define SECTION_SIZE_BITS 30
    > +#define SECTION_SIZE_BITS 27
    > #endif
    >
    > #endif
    > --
    > 2.15.0

    Thanks
    Barry

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