From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v6 06/26] mm: Add PG_arch_2 page flag
Date: Mon, 6 Jul 2020 12:21:13 +0100
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To: David Hildenbrand <david@redhat.com>
Cc: 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>, Andrew Morton <akpm@linux-foundation.org>, Steven Price <steven.price@arm.com>

On Mon, Jul 06, 2020 at 10:24:04AM +0200, David Hildenbrand wrote:
> On 03.07.20 17:36, Catalin Marinas wrote:
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 6be1aa559b1e..276140c94f4a 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -135,6 +135,9 @@ enum pageflags {
> >  #if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
> >  	PG_young,
> >  	PG_idle,
> > +#endif
> > +#ifdef CONFIG_64BIT
> > +	PG_arch_2,
> >  #endif
> >  	__NR_PAGEFLAGS,
> 
> People are usually *very* picky when it comes to new page flags. It
> somewhat concerns me that we bump up __NR_PAGEFLAGS for any 64bit arch.
> That feels wrong.

It was guarded by a specific config option initially but the comments
suggested that it could be dropped for 64-bit architectures:

https://lore.kernel.org/linux-arm-kernel/20200624113307.6165b3db2404c9d37b870a90@linux-foundation.org/

The page flags is indeed a pretty limited resource as it also includes
the sparsemem section, node and zone fields. However, on 64-bit this
should be fine (the sparsemem section is gone with vmemmap support).

-- 
Catalin