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

[Will Deacon]

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@xxxxxxxxxx>
> Signed-off-by: Baopeng Feng <fengbaopeng2@xxxxxxxxxxxxx>
> Signed-off-by: Xia Qing <saberlily.xia@xxxxxxxxxxxxx>
> ---
>  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?

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.

Will