Re: [PATCH] mm: Make PageType more efficient

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Mar 10, 2020 at 1:37 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Mar 10, 2020 at 01:17:17PM -0700, Alexander Duyck wrote:
> > On Tue, Mar 10, 2020 at 11:56 AM Matthew Wilcox <willy@infradead.org> wrote:
> > > PageType is a little hard for GCC to reason about, By checking
> > > ((~A) & flag) instead of (flag & (A | MASK) == MASK), GCC can do
> > > better optimisations, saving 652 bytes in page_alloc.o (which is
> > > a heavy user of PageBuddy).
> > >
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > ---
> > >  include/linux/page-flags.h | 6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > > index 1bf83c8fcaa7..8fc0876e2794 100644
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -725,14 +725,14 @@ PAGEFLAG_FALSE(DoubleMap)
> > >  #define PG_table       0x00000400
> > >  #define PG_guard       0x00000800
> > >
> > > -#define PageType(page, flag)                                           \
> > > -       ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> > > -

From what I can tell this is the only consumer of PAGE_TYPE_BASE.
Since it is removed you can probably remove that definition as well.

> > >  static inline int page_has_type(struct page *page)
> > >  {
> > >         return (int)page->page_type < PAGE_MAPCOUNT_RESERVE;
> > >  }
> > >
> > > +#define PageType(page, flag)                                           \
> > > +       (page_has_type(page) && (~page->page_type & flag))
> > > +

You can probably spare a cycle or two here by testing for
"!(page->page_type & flag)". That way you avoid the extra bit flipping
since the compiler can just handle the result of the AND op as it sees
fit.

> > >  #define PAGE_TYPE_OPS(uname, lname)                                    \
> > >  static __always_inline int Page##uname(struct page *page)              \
> > >  {                                                                      \
> >
> > If I recall all the page type is doing is clearing a single bit to
> > indicate the page type, and only one page type is supposed to be set
> > at a time correct?
> >
> > Is there any reason why we couldn't just do an addition and test?
> > Basically just add the flag + 1 and see if the value rolls over to 0.
> > I would think that would reduce to an even simpler setup since that
> > would be an addition with a test for carry flag or zero.
>
> I think we already allow for both PageKmemcg and PageTable to be set
> on the same page.  I don't want to stop people from being able to do
> combinations like that in the future.

Okay, i wasn't aware of that. So that prevents us from simplifying this further.