linux-kernel - Re: [PATCH] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS)

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Date:   Tue, 17 Aug 2021 21:44:36 -0700
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: introduce PAGEFLAGS_MASK to replace ((1UL <<
 NR_PAGEFLAGS) - 1)

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@...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@...edance.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)

Thanks!