Re: [PATCH] mm: Make PageType more efficient

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

On Tue, Mar 10, 2020 at 11:56 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> 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.