linux-kernel - Re: [PATCH v2 6/7] x86/Kconfig: Introduce ARCH_MHP_MEMMAP_ON_MEMORY

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Date:   Fri, 26 Feb 2021 13:04:51 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, VlastimilBabkavbabka@...e.cz,
        pasha.tatashin@...een.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2 6/7] x86/Kconfig: Introduce
 ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE

On Thu, Feb 25, 2021 at 07:29:07PM +0100, David Hildenbrand wrote:
> On 09.02.21 14:38, Oscar Salvador wrote:
> > Enable x86_64 platform to use the MHP_MEMMAP_ON_MEMORY feature.
> > 
> > Signed-off-by: Oscar Salvador <osalvador@...e.de>
> > ---
> >   arch/x86/Kconfig | 4 ++++
> >   1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 72663de8b04c..81046b7adb10 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -2440,6 +2440,10 @@ config ARCH_ENABLE_MEMORY_HOTREMOVE
> >   	def_bool y
> >   	depends on MEMORY_HOTPLUG
> > +config ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
> > +	def_bool y
> > +	depends on X86_64 && MEMORY_HOTPLUG && SPARSEMEM_VMEMMAP_ENABLE
> 
> It depends on SPARSEMEM_VMEMMAP, no?
> 
> I think we could have
> 
> SPARSEMEM_VMEMMAP_ENABLE=y
> and
> SPARSEMEM_VMEMMAP=n
> 
> on manually tuned configs.

I do not think this can happen:

from mm/Kconfig:

config SPARSEMEM_VMEMMAP_ENABLE
        bool

config SPARSEMEM_VMEMMAP
        bool "Sparse Memory virtual memmap"
        depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
        default y
        help
          SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
          pfn_to_page and page_to_pfn operations.  This is the most
          efficient option when sufficient kernel resources are available

and from arch/x86/Kconfig:

config ARCH_SPARSEMEM_ENABLE
        def_bool y
        depends on X86_64 || NUMA || X86_32 || X86_32_NON_STANDARD
        select SPARSEMEM_STATIC if X86_32
        select SPARSEMEM_VMEMMAP_ENABLE if X86_64


So, if I read this correctly, for SPARSEMEM_VMEMMAP to be true,
SPARSEMEM_VMEMMAP_ENABLE needs to be true as well.

Am I missing something?

-- 
Oscar Salvador
SUSE L3