Re: [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1)

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Jul 27, 2021 at 2:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Tue, Jul 27, 2021 at 5:04 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >
> > On 7/14/21 2:17 AM, 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 5922031ffab6..358d3f6fa976 100644
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -178,6 +178,8 @@ enum pageflags {
> > >       PG_reported = PG_uptodate,
> > >  };
> > >
> > > +#define PAGEFLAGS_MASK               (~((1UL << NR_PAGEFLAGS) - 1))
> >
> > Can you explain why you chose this definition instead of
> >
> > #define PAGEFLAGS_MASK          ((1UL << NR_PAGEFLAGS) - 1)
> >
> > and mostly use ~PAGEFLAGS_MASK below?
>
> Hi Mike,
>
> Actually, I learned from PAGE_MASK. So I thought the macro
> like xxx_MASK should be the format of 0x00...00ff...ff. I don't
                                           ^^^
Sorry. I mean 0xff...ff00...00 here.

> know if this is an unwritten rule. Please correct me if I am
> wrong.
>
> Thanks.
>
> > --
> > Mike Kravetz