On Tue, 10 Mar 2020 11:56:09 -0700 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
>
> 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).
>
> ...
>
> --- 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))
> +
`flag' should be parenthesized here. As should `page' if one is even
more paranoid.
I tried this:
--- a/include/linux/page-flags.h~mm-make-pagetype-more-efficient-fix
+++ a/include/linux/page-flags.h
@@ -731,7 +731,7 @@ static inline int page_has_type(struct p
}
#define PageType(page, flag) \
- (page_has_type(page) && (~page->page_type & flag))
+ (page_has_type(page) && !(page->page_type & ~(flag)))
#define PAGE_TYPE_OPS(uname, lname) \
static __always_inline int Page##uname(struct page *page) \
and page_alloc.o shrunk by 6782 bytes, half of it in
get_page_from_freelist(). Something crazy is going on. gcc-7.2.0.
