Re: [PATCH 2/5] mm: introduce save_page_flags to cooperate with show_page_flags

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Jul 27, 2021 at 7:18 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 7/14/21 2:17 AM, Muchun Song wrote:
> > Introduce save_page_flags to return the page flags which can cooperate
> > with show_page_flags. If we want to hihe some page flags from users, it
> > will be useful to alter save_page_flags directly. This is a preparation
> > for the next patch to hide some page flags from users.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/trace/events/mmflags.h  | 3 +++
> >  include/trace/events/page_ref.h | 8 ++++----
> >  2 files changed, 7 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> > index 390270e00a1d..69cb84b1257e 100644
> > --- a/include/trace/events/mmflags.h
> > +++ b/include/trace/events/mmflags.h
> > @@ -121,6 +121,9 @@ IF_HAVE_PG_IDLE(PG_idle,          "idle"          )               \
> >  IF_HAVE_PG_ARCH_2(PG_arch_2,         "arch_2"        )               \
> >  IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
> >
> > +#define save_page_flags(page)                                                \
> > +     (((page)->flags & ~PAGEFLAGS_MASK))
> > +
>
> Looking ahead to the next patch, this is changed to hide the PG_head
> flag for 'fake' head pages.
>
> IIRC, all vmemmap pages except the first will be mapped read only.  So,
> all vmemmap pages with 'fake' head pages will be read only.
>
> It seems that all the modified trace events below are associated with
> updates to page structs.  Therefore, it seems these events will never
> experience a 'fake' head page.  Am I missing something?

Totally right. I didn't realize this point before. We cannot see a fake page
struct in page refcount tracing. So this patch is definitely pointless. I'll
drop it in the next version. Thanks Mike.


>
> --
> Mike Kravetz