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Date: Wed, 24 Jun 2020 19:36:47 +0100
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v5 06/25] mm: Add PG_ARCH_2 page flag
Message-ID: <20200624183647.GU21350@casper.infradead.org>
References: <20200624175244.25837-1-catalin.marinas@arm.com>
 <20200624175244.25837-7-catalin.marinas@arm.com>
 <20200624113307.6165b3db2404c9d37b870a90@linux-foundation.org>
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Sender: owner-linux-mm@kvack.org
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Will Deacon <will@kernel.org>, Dave P Martin <Dave.Martin@arm.com>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Szabolcs Nagy <szabolcs.nagy@arm.com>, Kevin Brodsky <kevin.brodsky@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Peter Collingbourne <pcc@google.com>, Steven Price <steven.price@arm.com>
List-ID: <linux-arch.vger.kernel.org>
Message-ID: <20200624183647.Rz9tw_u_4YDWzKDVxx6tSBDbnYzH3Vl1yO4z0Vmsjpw@z>

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.