Re: [PATCHv2 3/4] mm: pack compound_dtor and compound_order into one word in struct page

[Kirill A. Shutemov]

On Tue, Aug 18, 2015 at 05:43:00PM +0200, Michal Hocko wrote:
> On Mon 17-08-15 18:09:04, Kirill A. Shutemov wrote:
> > The patch halves space occupied by compound_dtor and compound_order in
> > struct page.
> > 
> > For compound_order, it's trivial long -> int/short conversion.
> > 
> > For get_compound_page_dtor(), we now use hardcoded table for destructor
> > lookup and store its index in the struct page instead of direct pointer
> > to destructor. It shouldn't be a big trouble to maintain the table: we
> > have only two destructor and NULL currently.
> > 
> > This patch free up one word in tail pages for reuse. This is preparation
> > for the next patch.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> 
> Reviewed-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> [...]
> > @@ -145,8 +143,13 @@ struct page {
> >  						 */
> >  		/* First tail page of compound page */
> >  		struct {
> > -			compound_page_dtor *compound_dtor;
> > -			unsigned long compound_order;
> > +#ifdef CONFIG_64BIT
> > +			unsigned int compound_dtor;
> > +			unsigned int compound_order;
> > +#else
> > +			unsigned short int compound_dtor;
> > +			unsigned short int compound_order;
> > +#endif
> >  		};
> 
> Why do we need this ifdef? We can go with short for both 32b and 64b
> AFAICS.

My assumption was that operations on ints can be faster on some
[micro]arhictectures. I'm not sure if it's ever true.

> We do not use compound_order for anything else than the order, right?

Right.

> While I am looking at this, it seems we are jugling with type for order
> quite a lot - int, unsing int and even unsigned long.

Yeah. It's mess. I'll check if I can fix anything of it in v3.

-- 
 Kirill A. Shutemov
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