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

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Date:   Fri, 4 Dec 2020 13:44:00 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Will Deacon <will@...nel.org>
Cc:     Wei Li <liwei213@...wei.com>, catalin.marinas@....com,
        fengbaopeng2@...ilicon.com, nsaenzjulienne@...e.de,
        steve.capper@....com, song.bao.hua@...ilicon.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        butao@...ilicon.com
Subject: Re: [PATCH] arm64: mm: decrease the section size to reduce the
 memory reserved for the page map

On Fri, Dec 04, 2020 at 11:13:47AM +0000, Will Deacon wrote:
> On Fri, Dec 04, 2020 at 09:44:43AM +0800, Wei Li wrote:
> > 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>
> > ---
> >  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
> 
> We chose '30' to avoid running out of bits in the page flags. What changed?

I think that for 64-bit there are still plenty of free bits. I didn't
check now, but when I played with SPARSEMEM on m68k there were 8 bits
for section out of 32.

> With this patch, I can trigger:
> 
> ./include/linux/mmzone.h:1170:2: error: Allocator MAX_ORDER exceeds SECTION_SIZE
> #error Allocator MAX_ORDER exceeds SECTION_SIZE
> 
> if I bump up NR_CPUS and NODES_SHIFT.

I don't think it's related to NR_CPUS and NODES_SHIFT. 
This seems rather 64K pages that cause this.

Not that is shouldn't be addressed.

> Will

-- 
Sincerely yours,
Mike.