Re: [PATCH] mm: Make PageType more efficient

[Matthew Wilcox <willy@infradead.org>]

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)
> > -
> >  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))
> > +
> >  #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.