On Tue, Dec 19, 2017 at 09:19:56AM +0100, Michal Hocko wrote:
> On Sat 16-12-17 08:44:23, Matthew Wilcox wrote:
> > From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> >
> > Neither of these values get even close to 256; compound_dtor is
> > currently at a maximum of 3, and compound_order can't be over 64.
> > No machine has inefficient access to bytes since EV5, and while
> > those are still supported, we don't optimise for them any more.
>
> Hmm, so the improvement is the ifdef-ery removale, right? Beucase this
> will not shrink the structure size AFAICS. I think that the former is
> a sufficient justification. Maybe you should spell it out.
I'll add that to the changelog. It also frees up 2-6 bytes for another
usage, if we decide there's something else we need to store in a
compound page. I also added the pahole diff output:
@@ -34,8 +34,8 @@
struct callback_head callback_head; /* 32 16 */
struct {
long unsigned int compound_head; /* 32 8 */
- unsigned int compound_dtor; /* 40 4 */
- unsigned int compound_order; /* 44 4 */
+ unsigned char compound_dtor; /* 40 1 */
+ unsigned char compound_order; /* 41 1 */
}; /* 32 16 */
}; /* 32 16 */
union {
> > Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> > ---
> > include/linux/mm_types.h | 15 ++-------------
> > 1 file changed, 2 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 5521c9799c50..1a3ba1f1605d 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -136,19 +136,8 @@ struct page {
> > unsigned long compound_head; /* If bit zero is set */
> >
> > /* First tail page only */
> > -#ifdef CONFIG_64BIT
> > - /*
> > - * On 64 bit system we have enough space in struct page
> > - * to encode compound_dtor and compound_order with
> > - * unsigned int. It can help compiler generate better or
> > - * smaller code on some archtectures.
> > - */
> > - unsigned int compound_dtor;
> > - unsigned int compound_order;
> > -#else
> > - unsigned short int compound_dtor;
> > - unsigned short int compound_order;
> > -#endif
> > + unsigned char compound_dtor;
> > + unsigned char compound_order;
> > };
> >
> > #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS
> > --
> > 2.15.1
> >
> > --
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> --
> Michal Hocko
> SUSE Labs
>
> --
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