Re: [PATCH] mm: Make PageType more efficient

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 10, 2020 at 06:10:27PM -0700, Andrew Morton wrote:
> On Tue, 10 Mar 2020 11:56:09 -0700 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).
> > 
> > ...
> >
> > --- 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 suppose so.  The thing is, the flag name isn't specified by the user;
users say 'if (PageBuddy(page))' and the flag gets expanded through
PAGE_TYPE_OPS.  We also don't need to worry about the user doing 'if
(PageBuddy(page++))' as it'll expand into a call to

static __always_inline int PageBuddy(struct page *page)
{
	return PageType(page, PG_buddy)
}

> 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)))

Mr Boole would like to let you know you misapplied his rules of algebra.
~flag is going to expand into 0xfffffff7f and then gcc will figure out
the expression as a whole is always false, and so no page is ever a
buddy page, and ..

> 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.