linux-kernel - RE: [PATCH] arm64: mm: decrease the section size to reduce the memory reserved for the page map

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Message-ID: <731f9389df474a968b2799fa698821da@hisilicon.com>
Date:   Fri, 4 Dec 2020 01:50:40 +0000
From:   "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
To:     "liwei (CM)" <liwei213@...wei.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "will@...nel.org" <will@...nel.org>
CC:     fengbaopeng <fengbaopeng2@...ilicon.com>,
        "nsaenzjulienne@...e.de" <nsaenzjulienne@...e.de>,
        "steve.capper@....com" <steve.capper@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        butao <butao@...ilicon.com>
Subject: RE: [PATCH] arm64: mm: decrease the section size to reduce the memory
 reserved for the page map



> -----Original Message-----
> From: liwei (CM)
> Sent: Friday, December 4, 2020 2:45 PM
> To: catalin.marinas@....com; rppt@...ux.ibm.com; will@...nel.org; liwei (CM)
> <liwei213@...wei.com>
> Cc: fengbaopeng <fengbaopeng2@...ilicon.com>; nsaenzjulienne@...e.de;
> steve.capper@....com; Song Bao Hua (Barry Song) <song.bao.hua@...ilicon.com>;
> linux-arm-kernel@...ts.infradead.org; linux-kernel@...r.kernel.org; butao
> <butao@...ilicon.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@...wei.com>
> Signed-off-by: Baopeng Feng <fengbaopeng2@...ilicon.com>
> Signed-off-by: Xia Qing <saberlily.xia@...ilicon.com>
> ---

Reviewed-by: Barry Song <song.bao.hua@...ilicon.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