Re: [PATCH 1/2] mm: Introduce common STRUCT_PAGE_MAX_SHIFT define

[Andrew Morton]

On Wed,  7 Nov 2018 10:38:58 -0700 Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote:

> This define is used by arm64 to calculate the size of the vmemmap
> region. It is defined as the log2 of the upper bound on the size
> of a struct page.
> 
> We move it into mm_types.h so it can be defined properly instead of
> set and checked with a build bug. This also allows us to use the same
> define for riscv.
> 
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -34,15 +34,6 @@
>   */
>  #define PCI_IO_SIZE		SZ_16M
>  
> -/*
> - * Log2 of the upper bound of the size of a struct page. Used for sizing
> - * the vmemmap region only, does not affect actual memory footprint.
> - * We don't use sizeof(struct page) directly since taking its size here
> - * requires its definition to be available at this point in the inclusion
> - * chain, and it may not be a power of 2 in the first place.
> - */
> -#define STRUCT_PAGE_MAX_SHIFT	6

Well that was lame.

> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -206,6 +206,11 @@ struct page {
>  #endif
>  } _struct_page_alignment;
>  
> +/*
> + * Used for sizing the vmemmap region on some architectures
> + */
> +#define STRUCT_PAGE_MAX_SHIFT	(order_base_2(sizeof(struct page)))

Much better.

Acked-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>