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

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Date:   Wed, 18 Aug 2021 12:35:08 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Roman Gushchin <guro@...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 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.

Thanks.

>
> Thanks!
>
> > +
> >  #ifndef __GENERATING_BOUNDS_H
> >
> >  static inline unsigned long _compound_head(const struct page *page)
> > @@ -975,7 +977,7 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
> >   * alloc-free cycle to prevent from reusing the page.
> >   */
> >  #define PAGE_FLAGS_CHECK_AT_PREP     \
> > -     (((1UL << NR_PAGEFLAGS) - 1) & ~__PG_HWPOISON)
> > +     (~(PAGEFLAGS_MASK | __PG_HWPOISON))
> >
> >  #define PAGE_FLAGS_PRIVATE                           \
> >       (1UL << PG_private | 1UL << PG_private_2)
> > diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
> > index 5d2ea93956ce..643b1b4e9f27 100644
> > --- a/include/trace/events/page_ref.h
> > +++ b/include/trace/events/page_ref.h
> > @@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
> >
> >       TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
> >               __entry->pfn,
> > -             show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
> > +             show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
> >               __entry->count,
> >               __entry->mapcount, __entry->mapping, __entry->mt,
> >               __entry->val)
> > @@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
> >
> >       TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
> >               __entry->pfn,
> > -             show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
> > +             show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
> >               __entry->count,
> >               __entry->mapcount, __entry->mapping, __entry->mt,
> >               __entry->val, __entry->ret)
> > diff --git a/lib/test_printf.c b/lib/test_printf.c
> > index 8ac71aee46af..eadcc6458b10 100644
> > --- a/lib/test_printf.c
> > +++ b/lib/test_printf.c
> > @@ -614,7 +614,7 @@ page_flags_test(int section, int node, int zone, int last_cpupid,
> >       bool append = false;
> >       int i;
> >
> > -     flags &= BIT(NR_PAGEFLAGS) - 1;
> > +     flags &= ~PAGEFLAGS_MASK;
> >       if (flags) {
> >               page_flags |= flags;
> >               snprintf(cmp_buf + size, BUF_SIZE - size, "%s", name);
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index dd006adfe853..ff7c180a2971 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -2019,7 +2019,7 @@ static const struct page_flags_fields pff[] = {
> >  static
> >  char *format_page_flags(char *buf, char *end, unsigned long flags)
> >  {
> > -     unsigned long main_flags = flags & (BIT(NR_PAGEFLAGS) - 1);
> > +     unsigned long main_flags = flags & ~PAGEFLAGS_MASK;
> >       bool append = false;
> >       int i;
> >
> > --
> > 2.11.0
> >