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Date:   Wed, 18 Aug 2021 00:39:19 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH] mm: introduce PAGEFLAGS_MASK to replace ((1UL <<
 NR_PAGEFLAGS) - 1)
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On Tue, 17 Aug 2021 21:44:36 -0700 Roman Gushchin <guro@fb.com> wrote:

> On Wed, Aug 18, 2021 at 12:35:08PM +0800, Muchun Song wrote:
> > On Wed, Aug 18, 2021 at 10:16 AM Roman Gushchin <guro@fb.com> wrote:
> > >
> > > On Tue, Aug 17, 2021 at 11:30:32AM +0800, Muchun Song wrote:
> > > > Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing
> > > > PAGEFLAGS_MASK to make the code clear to get the page flags.
> > > >
> > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > > ---
> > > >  include/linux/page-flags.h      | 4 +++-
> > > >  include/trace/events/page_ref.h | 4 ++--
> > > >  lib/test_printf.c               | 2 +-
> > > >  lib/vsprintf.c                  | 2 +-
> > > >  4 files changed, 7 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > > > index 54c4af35c628..1f951ac24a5e 100644
> > > > --- a/include/linux/page-flags.h
> > > > +++ b/include/linux/page-flags.h
> > > > @@ -180,6 +180,8 @@ enum pageflags {
> > > >       PG_reported = PG_uptodate,
> > > >  };
> > > >
> > > > +#define PAGEFLAGS_MASK               (~((1UL << NR_PAGEFLAGS) - 1))
> > >
> > > Hm, isn't it better to invert it? Like
> > > #define PAGEFLAGS_MASK          ((1UL << NR_PAGEFLAGS) - 1)
> > >
> > > It feels more usual and will simplify the rest of the patch.
> > 
> > Actually, I learned from PAGE_MASK. So I thought the macro
> > like xxx_MASK should be the format of 0xff...ff00...00. I don't
> > know if it is an unwritten rule. I can invert PAGEFLAGS_MASK
> > if it's not a rule.
> 
> There are many examples of both approached in the kernel tree,
> however I'd say the more common is without "~" (out of my head).
> 
> It's definitely OK to define it like
> #define PAGEFLAGS_MASK          ((1UL << NR_PAGEFLAGS) - 1)
> 

PAGE_MASK has always seemed weird to me but I figured that emulating it
would be the approach of least surprise.  Might be wrong about that...