[PATCH v5 06/25] mm: Add PG_ARCH_2 page flag

[Andrew Morton]

On Wed, 24 Jun 2020 18:52:25 +0100 Catalin Marinas <catalin.marinas at arm.com> wrote:

> From: Steven Price <steven.price at arm.com>
> 
> For arm64 MTE support it is necessary to be able to mark pages that
> contain user space visible tags that will need to be saved/restored e.g.
> when swapped out.
> 
> To support this add a new arch specific flag (PG_ARCH_2) that arch code
> can opt into using ARCH_USES_PG_ARCH_2.
> 
> ...
>
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -217,6 +217,9 @@ u64 stable_page_flags(struct page *page)
>  	u |= kpf_copy_bit(k, KPF_PRIVATE_2,	PG_private_2);
>  	u |= kpf_copy_bit(k, KPF_OWNER_PRIVATE,	PG_owner_priv_1);
>  	u |= kpf_copy_bit(k, KPF_ARCH,		PG_arch_1);
> +#ifdef CONFIG_ARCH_USES_PG_ARCH_2
> +	u |= kpf_copy_bit(k, KPF_ARCH_2,	PG_arch_2);
> +#endif

Do we need CONFIG_ARCH_USES_PG_ARCH_2?  What would be the downside to
giving every architecture a PG_arch_2, but only arm64 uses it (at
present)?