linux-kernel - Re: [PATCH v2] arm64: add ARM64-specific support for flatmem

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Date:   Fri, 6 Jul 2018 17:16:22 +0000
From:   "Nikunj Kela (nkela)" <nkela@...co.com>
To:     Will Deacon <will.deacon@....com>
CC:     Catalin Marinas <catalin.marinas@....com>,
        "xe-kernel@...ernal.cisco.com" <xe-kernel@...ernal.cisco.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] arm64: add ARM64-specific support for flatmem

I might be wrong but I was under the impression that Sparse memory is only of use on NUMA systems or systems supporting hot pluggable memory. the NODE_DATA() macro needs to be switched on NUMA, but not on FLATMEM. I borrowed this dependency from x86 Kconfig. If this is not needed, I will remove it.

Thanks,
-Nikunj

On 7/6/18, 10:05 AM, "Will Deacon" <will.deacon@....com> wrote:

    On Fri, Jul 06, 2018 at 10:02:15AM -0700, Nikunj Kela wrote:
    > Flatmem is useful in reducing kernel memory usage.
    > One usecase is in kdump kernel. We are able to save
    > ~14M by moving to flatmem scheme.
    > 
    > Cc: xe-kernel@...ernal.cisco.com
    > Cc: Nikunj Kela <nkela@...co.com>
    > Signed-off-by: Nikunj Kela <nkela@...co.com>
    > ---
    >  arch/arm64/Kconfig | 4 ++++
    >  1 file changed, 4 insertions(+)
    > 
    > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
    > index 42c090c..96ab9a7 100644
    > --- a/arch/arm64/Kconfig
    > +++ b/arch/arm64/Kconfig
    > @@ -775,6 +775,10 @@ config ARCH_SPARSEMEM_DEFAULT
    >  config ARCH_SELECT_MEMORY_MODEL
    >  	def_bool ARCH_SPARSEMEM_ENABLE
    >  
    > +config ARCH_FLATMEM_ENABLE
    > +	def_bool y
    > +	depends on !NUMA
    
    I still don't understand what this has to do with NUMA. SPARSEMEM is still
    going to be what you want on a non-NUMA system with large holes in the
    physical memory map, no?
    
    Will