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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jun 24, 2020 at 11:33:07AM -0700, Andrew Morton wrote:
> On Wed, 24 Jun 2020 18:52:25 +0100 Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> > From: Steven Price <steven.price@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)?

32-bit architectures don't have space for it.  We could condition it on
CONFIG_64BIT instead.