On Wed, Mar 10, 2021 at 11:41 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 08-03-21 18:28:07, Muchun Song wrote:
> > When the "struct page size" crosses page boundaries we cannot
> > make use of this feature. Let free_vmemmap_pages_per_hpage()
> > return zero if that is the case, most of the functions can be
> > optimized away.
>
> I am confused. Don't you check for this in early_hugetlb_free_vmemmap_param already?
Right.
> Why do we need any runtime checks?
If the size of the struct page is not power of 2, compiler can think
is_hugetlb_free_vmemmap_enabled() always return false. So
the code snippet of this user can be optimized away.
E.g.
if (is_hugetlb_free_vmemmap_enabled())
/* do something */
The compiler can drop "/* do something */" directly, because
it knows is_hugetlb_free_vmemmap_enabled() always returns
false.
Thanks.
>
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> > Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> > Tested-by: Chen Huang <chenhuang5@xxxxxxxxxx>
> > Tested-by: Bodeddula Balasubramaniam <bodeddub@xxxxxxxxxx>
> > ---
> > include/linux/hugetlb.h | 3 ++-
> > mm/hugetlb_vmemmap.c | 7 +++++++
> > mm/hugetlb_vmemmap.h | 6 ++++++
> > 3 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index c70421e26189..333dd0479fc2 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -880,7 +880,8 @@ extern bool hugetlb_free_vmemmap_enabled;
> >
> > static inline bool is_hugetlb_free_vmemmap_enabled(void)
> > {
> > - return hugetlb_free_vmemmap_enabled;
> > + return hugetlb_free_vmemmap_enabled &&
> > + is_power_of_2(sizeof(struct page));
> > }
> > #else
> > static inline bool is_hugetlb_free_vmemmap_enabled(void)
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 33e42678abe3..1ba1ef45c48c 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -265,6 +265,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> > BUILD_BUG_ON(__NR_USED_SUBPAGE >=
> > RESERVE_VMEMMAP_SIZE / sizeof(struct page));
> >
> > + /*
> > + * The compiler can help us to optimize this function to null
> > + * when the size of the struct page is not power of 2.
> > + */
> > + if (!is_power_of_2(sizeof(struct page)))
> > + return;
> > +
> > if (!hugetlb_free_vmemmap_enabled)
> > return;
> >
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > index cb2bef8f9e73..29aaaf7b741e 100644
> > --- a/mm/hugetlb_vmemmap.h
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -21,6 +21,12 @@ void hugetlb_vmemmap_init(struct hstate *h);
> > */
> > static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > {
> > + /*
> > + * This check aims to let the compiler help us optimize the code as
> > + * much as possible.
> > + */
> > + if (!is_power_of_2(sizeof(struct page)))
> > + return 0;
> > return h->nr_free_vmemmap_pages;
> > }
> > #else
> > --
> > 2.11.0
> >
>
> --
> Michal Hocko
> SUSE Labs
